Term
| What is the difference between menstrual age and embryologic age? |
|
Definition
| All embryonic dates in this chapter reflect menstrual age rather than embryologic age. Menstrual age (also known as gestational age) is calculated by adding 2 weeks (14 days) to embryologic age. Menstrual age refers to length of time calculated from the first day of the last normal menstrual period (LMP) to the point at which the pregnancy is being assessed. |
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Term
| Describe fertilization from the point at which the ovum is swept into the tube to the development of the yolk sac. |
|
Definition
The fertilized ovum, which should now be referred to as a zygote, undergoes rapid cellular division to form the 16-cell morula. Further cell proliferation brings the morula to the blastocyst stage, which contains trophoblastic cells and the “inner cell mass,” which forms the embryo. The blastocyst typically enters the uterus 4 to 5 days after fertilization, with implantation occurring 7 to 9 days after ovulation. During implantation, proteolytic enzymes produced by the trophoblasts erode endometrial mucosa and maternal capillaries, resulting in a primitive blood exchange network between mother and conceptus.
When implantation is completed the trophoblast goes on to form primary villi, which initially circumvent the early gestational sac, giving it the sonographic ring appearance. Within the conceptus the inner cell mass matures into the bilaminar embryonic disc, the future embryo, and the primary yolk sac. At approximately 23 days menstrual age the primary yolk sac is pinched off by the extra embryonic coelom, forming the secondary yolk sac. The secondary yolk sac is the yolk sac seen, sonographically, throughout the first trimester. The amniotic and chorionic cavities also develop and evolve during this period of gestation. |
|
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Term
| What level does the hCG need to be to demonstrate a normal gestational sac? |
|
Definition
| A normal gestational sac can be consistently demonstrated when the hCG level is 1800 mIU/ml (Second International Standard) or greater when using transabdominal sonography. |
|
|
Term
| What is the purpose of the secondary yolk sac? |
|
Definition
The secondary or sonographic yolk sac has essential functions in embryonic development, including: 1. provision of nutrients to the developing embryo;
2. hematopoiesis; and 3. development of embryonic endoderm, which forms the primitive gut. |
|
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Term
| When does the fetal skeletal system begin to develop? |
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Definition
| The skeletal system begins to develop during the sixth week with the upper limbs forming first, followed by the lower extremities. The hands and feet develop later in the first trimester and are completed by the end of the 10th week of gestation. Sonographically, limb buds can be detected, generally, from the seventh week on; the fingers and toes are recognizable at 11 weeks using EVS. |
|
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Term
| Describe what happens to the bowel migration in the first trimester. |
|
Definition
*By week 12 gastro. completed on the right side.
The anterior abdominal wall is developed by 6 weeks of gestation from the fusion of four ectomesodermal body folds. Simultaneously, the primitive gut is formed as a result of the incorporation of the dorsal yolk sac into the embryo. The midgut, derived from the primitive gut, develops and forms the majority of the small bowel, cecum, ascending colon, and proximal transverse colon. Since the midgut is in direct communication with the yolk sac, amniotic cavity expansion pulls the yolk sac away from the embryo forming the yolk stalk. As amniotic expansion occurs, the midgut elongates faster than the embryo is growing, causing the midgut to herniate into the base of the umbilical cord. Until approximately 10 weeks gestation, the midgut loop continues to grow and rotate before it descends into the fetal abdomen at about the eleventh week. |
|
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Term
| Name the three primary brain vesicles that develop after the sixth week of gestation. |
|
Definition
| Around the sixth week of gestation, three primary brain vesicles develop: the prosencephalon, mesencephalon, and rhombencephalon. Because of rapid cell proliferation in relation to cranial vault space, flexures of the developing brain occur. |
|
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Term
| Describe the development of the choriod plexus and cerebral ventricles in the later part of the first trimester. |
|
Definition
*Lateral vent. size is >10mm
The cerebral hemispheres may be seen at around 9 weeks’ gestation. The echogenic choroid plexus, which fills the lateral cerebral ventricles, can be visualized. Sonolucent cerebrospinal fluid can be demarcated around the choroid plexus. It is important to note that the lateral ventricles completely fill the cerebral vault at this time in gestation. The cerebral hemispheres are relatively small compared with the rest of the brain, although this relationship rapidly changes at the beginning of the second trimester. |
|
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Term
| What is the first organ to function in the embryo? |
|
Definition
|
|
Term
| Describe the normal early cardiac activity. |
|
Definition
| Embryonic cardiac activity should always be seen by 46 menstrual days. Embryonic cardiac rates vary with gestational age. Rates of 90 bpm at 6 weeks increase to rates of 170 bpm at 9 weeks, with rates of approximately 140 bpm through the remainder of the first and second trimesters. |
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Term
| How is the mean sac diameter determined? |
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Definition
| Sonographically, the gestational sac size or mean sac diameter is determined by the average sum of the length, width, and height of the gestational sac. These measurements are obtained in both sagittal and coronal/semicoronal sonographic planes. When measuring the mean sac diameter, the sonographer should only measure gestational sac fluid space, not including the echogenic decidua. |
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Term
| How closely does visualization of the yolk sac predict a viable pregnancy? |
|
Definition
| Visualization of the yolk sac predicts a viable pregnancy in over 90% of cases. Conversely, failure to visualize the yolk sac, with a minimum of 8 mm MSD, using EVS, should provoke suspicion of abnormal pregnancy. TA studies have shown that the yolk sac should be seen within mean sac diameters of 10 to 15 mm and always should be visualized with a mean sac diameter of 20 mm. |
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Term
| What is the sonographic appearance of a dichorionic and diamniotic pregnancy in the first trimester? |
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Definition
| Dizygotic (two ova) twin pregnancies, which comprise 70% of all twins, are by definition dichorionic and diamniotic. Sonographically, dichorionic and diamniotic twins appear as two separate gestational sacs with individual trophoblastic tissue, which allows the appearance of a thick dividing membrane. As pregnancy progresses, this membrane becomes thinner secondary to the diminished space between the two sacs, so this diagnosis may be more difficult later in gestation. In a dichorionic–diamniotic pregnancy, each sac has an individual yolk sac, amniotic membrane, and embryo. |
|
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Term
| Name the most likely cranial abnormalities that can be diagnosed with ultrasound in the first trimester. |
|
Definition
| The dominant structure seen within the embryonic cranium within the first trimester is that of the choroid plexus, which fills the lateral ventricles that in turn fill the cranial vault. Thus, the diagnosis of hydrocephalus in the first trimester is impossible. However, anomalies of cranial organization, such as holoprosencephaly, have been described in the first trimester. The previously described rhombencephalon–hindbrain is a cystic structure within the posterior aspect of the embryonic cranium that should not be confused with abnormality. A diagnosis of hydranencephaly (brain necrosis from occlusion of the internal carotid arteries) is sonographically demonstrated loss of all intracranial anatomy. In the first trimester, anencephaly should be diagnosed with caution. Reports have shown normal amounts of brain matter seen in the first trimester embryo with an anencephaly, unlike classic sonographic appearances in the second and third trimesters. Ossification of the cranial vault is not complete in the first trimester; the resulting false cranial border definition may give rise to a false-negative diagnosis. Extreme caution is advised if an embryonic cranial abnormality is suspected. Because traditional cranial anatomy can be visualized after 12 to 14 weeks gestation, repeat the sonogram at this time to either confirm or rule out abnormality. |
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Term
| Where does the cystic hygroma first appear? |
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Definition
| Cystic hygroma visualized in the first trimester may vary in size, but all appear on the posterior aspect of the fetal neck and upper thorax. Soft tissue thickening also may be present and should be considered as nuchal thickening. Although cystic hygroma and nuchal thickening may be concordant, differentiation may be difficult. A potential diagnostic pitfall for the sonographer is misinterpreting the hypoechoic or sonolucent embryonic skin surface in the region of the posterior neck. This has been described as the pseudomembrane sign and should not be confused with cystic hygroma, encephalocele, cervical meningomyelocele, teratoma, or hemangioma. Caution also should be observed in differentiating the pseudomembrane from the normal amniotic membrane on which the embryo is lying. |
|
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Term
| What is the most common ovarian mass seen in the first trimester of pregnancy? |
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Definition
|
|
Term
| Most common location of an ectopic pregnancy. |
|
Definition
| Ectopic pregnancy occurs within the fallopian tube in approximately 95% of patients. |
|
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Term
| How can the sonographer distinguish a complete versus incomplete abortion? |
|
Definition
Characteristics for the sonographic diagnosis of complete abortion are of an empty uterus with no adnexal masses or free fluid and positive hCG levels. Serial hCG levels demonstrate rapid decline. Caution should be taken when a positive pregnancy test and an empty uterus are seen, given the possibility that an early normal intrauterine pregnancy between 3 and 5 weeks may be present. Consequently, serial hCG levels always should be obtained.
Incomplete abortion may show several sonographic findings, ranging from an intact gestational sac with a nonliving embryo to a collapsed gestational sac with gross misshaping. Often women who are clinically undergoing abortion or have had elective termination require follow-up sonography to determine if retained products of conception are present. Sonography of retained products may be subtle; a thickened endometrium greater than 5 mm may be the only sonographic evidence of such diagnosis. Obvious embryonic parts, which may or may not cause acoustic shadowing, are obvious evidence of retained products of conception. |
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Term
| What are the clinical signs and sonographic findings in gestational trophoblastic disease? |
|
Definition
Gestational trophoblastic disease is a proliferative disease of the trophoblast after a pregnancy. It represents a spectrum of disease from a relatively benign form, hydatidiform mole, to a more malignant form, invasive mole, or choriocarcinoma. The clinical hallmark of gestational trophoblastic disease is vaginal bleeding in the first or early second trimester. The serum levels of beta-hCG are dramatically elevated, often greater than 100,000 IU/ml. The patient also may experience symptoms of hyperemesis gravidarum or preeclampsia.
The sonographic appearance of molar pregnancy varies with gestational age. The characteristic “snowstorm” appearance of hydatidiform mole, which includes a moderately echogenic soft tissue mass filling the uterine cavity and studded with small cystic spaces representing hydropic chorionic villi, may only be specific for a second trimester mole. Sonographic identification of the disease in the first trimester has been considered difficult. The appearance of first trimester molar pregnancy may simulate a missed abortion, incomplete abortion, blighted ovum, or hydropic degeneration of the placenta associated with missed abortion. It also may be seen as a small echogenic mass filling the uterine cavity without the characteristic vesicles. The vesicles or cystic spaces may be too small to be seen by ultrasound in the first trimester. |
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Term
| Name the six functions of amniotic fluid. |
|
Definition
• To act as a cushion to protect the fetus
• To allow embryonic and fetal movements
• To prevent adherence of the amnion to the embryo
• To allow symmetric growth
• To maintain a constant temperature
• To act as a reservoir to fetal metabolites before their excretion by the maternal system |
|
|
Term
| Explain the trimester system in pregnancy. |
|
Definition
| Pregnancy is divided into trimesters or thirds. The first trimester covers the first week of pregnancy through the 12th week, the second trimester continues the 13th week through the 26th week, and the third trimester commences with the 27th week and concludes at the 40th week of pregnancy. A pregnancy extending beyond the 42nd week is considered a postterm or postdate gestation. |
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Term
| Describe the gravidity and parity clinical labeling of pregnancy. |
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Definition
| The sonographer should understand the clinical labeling of pregnancy. Pregnancy history includes gravidity (G) (number of pregnancies, including the present one) and parity (P). Parity is described using a numeric system describing all pregnancy outcomes. The numeric sequence, P0000, is commonly used. Numbers are assigned to follow the parity symbol, P. The numbers represent, in order, full-term pregnancies, premature births, abortions, and living children. For instance, a G4P2103 describes a patient undergoing her fourth pregnancy. She has had two full-term pregnancies, one premature birth, no abortions, and has three living children. |
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Term
| How is situs determined in the fetus? |
|
Definition
The right and left sides of the fetus should be determined to ensure normal situs (positioning) of fetal organs. For instance, if the fetus is in a vertex presentation with the fetal spine toward the maternal right side, the right side of the fetus is down while the left side is up.
One should further differentiate the right from left sides by identifying anatomic landmarks. For example, the fetal stomach lies on the fetal left side, gallbladder on the right side, and apex of the heart toward the fetal left side. The fetal aorta lies slightly to the left of midline, anterior to the spine, whereas the IVC is to the right of midline and slightly more anterior to the aorta. |
|
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Term
| What is the function of the ductus venosus in fetoplacental circulation? |
|
Definition
| A special vascular connection, the ductus venosus, carries oxygen-rich blood from the umbilical vein directly to the IVC, which empties directly into the right atrium. This blood bypasses the liver. |
|
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Term
| Describe the anatomy and function of the umbilical vein. |
|
Definition
| The normal human umbilical cord contains an umbilical vein and two umbilical arteries. The umbilical vein transports oxygenated blood from the placenta, whereas the paired umbilical arteries return deoxygenated blood from the fetus to the placenta for purification. |
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Term
| Why is the production of amniotic fluid critical to fetal development? |
|
Definition
| Amniotic fluid serves several important functions during intrauterine life. Amniotic fluid allows the fetus to move freely within the amniotic cavity while maintaining intrauterine temperature and protecting the developing fetus from injury. |
|
|
Term
| Describe the earliest sonographic evidence of an intrauterine pregnancy, including the sonographic appearance, location, and biochemical laboratory values. |
|
Definition
| The earliest sonographic findings of an intrauterine pregnancy are thickening of the decidua. Sonographically, this appears as an echogenic, thickening filling of the fundal region of the endometrial cavity. It occurs at approximately 3 to 4 weeks’ gestation. At approximately 4 weeks menstrual age, a small hypoechoic area appears in the fundus or midportion of the uterus known as the double decidual sac sign. As the sac embeds further into the uterus, it is surrounded by an echogenic rim and is seen within the choriodecidual tissue. This is known as the chorionic or gestational sac. At 5 weeks the average internal diameter of the gestational sac, calculated as the mean of the anteroposterior diameter, transverse diameter, and longitudinal diameter, can provide an adequate estimation of menstrual age. A gestational sac should be seen within the uterine cavity when the beta human chorionic gonadotropin (BhCG) is above 500 mIU/ml (second international standard). This becomes especially important when evaluating a pregnancy for ectopic implantation. |
|
|
Term
| What is the function of the yolk sac? Describe its sonographic appearance and size. |
|
Definition
| When the gestational sac exceeds 8 mm in mean internal diameter, a yolk sac should be seen. The yolk sac is identified as a small, anechoic circular structure within the gestational sac. It provides early transfer of nutrients from the trophoblast to the embryo. It also aids in the early formation of the primitive gut and vitelline arteries and veins, and the production of the primordial germ cells. Yolk sac size has not been correlated with gestational age determination, but the size, shape, and number of yolk sacs have been well described. Normal yolk sac size should be less than 6 mm; over 8 mm has been associated with poor pregnancy outcome. |
|
|
Term
Describe the anatomic landmarks used to obtain a biparietal diameter and the
technique used to accurately measure it. |
|
Definition
| Obtain BPD of the fetal head at the transverse level of the midbrain; falx, septum pellucidum, and thalamic nuclei. Make sure the head is symmetric and oval. Measure from the outer to inner margins of the skull. In the third trimester, the BPD is not as accurate in predicting fetal age; it may approach ± 3 to 3.5 weeks. |
|
|
Term
| What is a cephalic index and what is the range of normal? |
|
Definition
The two most frequently noted alterations in head shape are dolichocephaly and brachycephaly. In dolichocephaly the head is shortened in the transverse plane (BPD) and elongated in the anteroposterior plane (OFD). In brachycephaly the head is elongated in the transverse diameter (BPD) and shortened in the anteroposterior diameter (OFD). One can underestimate gestational age from a dolichocephalic head or overestimate with brachycephaly. Because of these variations in fetal head shape, a cephalic index (CI) has been devised to determine the normality of the fetal head shape: CI = BPD/OFD/100.
A normal cephalic index is 80% |m1 standard deviation. The range of normal is 75% to 85%. A CI of greater than 85% suggests brachycephaly, and of less than 75% suggests dolichocephaly. |
|
|
Term
| Discuss the various orbital measurements and how they are obtained. |
|
Definition
| The ocular distance (OD), binocular distance (BD), and interocular distance (IOD) can be measured. The fetal orbit should be measured in a plane slightly more caudal than the BPD. The orbits are accessible in every head position except the occipitoanterior position (i.e., face looking down). All measurements should be taken from outer border to outer border. The OD measures a single fetal orbit. The BD includes both fetal orbits at the same time, whereas the IOD measures the length between the two orbits. |
|
|
Term
| Describe the two techniques used to measure a fetal cerebellum. What is the classic shape and why is the shape important? |
|
Definition
| The cerebellum can be measured from the same level that the BPD is obtained, angling back into the posterior fossa to include the full length of the cerebellum. The cerebellum should have a dumbbell shape. The widest diameter of the cerebellum should be measured. The banana and lemon signs associate an abnormally shaped cerebellum with fetal spina bifida. In the presence of spina bifida, the fetal cerebellum is pulled downward into the foramen magnum, altering its shape to appear oblong or banana shaped. The frontal bones of the fetal skull also give in to this pressure, collapsing and giving the fetal head a lemon shape. |
|
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Term
| Describe the difference between symmetric and asymmetric IUGR. |
|
Definition
Symmetric growth restriction is characterized by a fetus that is small in all physical parameters (e.g., BPD, HC, AC, FL). This is usually the result of a severe insult in the first trimester. The causes may include low genetic growth potential, intrauterine infection, severe maternal malnutrition, fetal alcohol syndrome, chromosomal anomaly, or severe congenital anomaly.
Asymmetric growth restriction is the more common IUGR and is usually caused by placental insufficiency. This may be the result of maternal disease such as diabetes (classes D–F) or chronic hypertension, cardiac or renal disease, abruptio placenta, multiple pregnancy, smoking, poor weight gain, drug usage, or uterine anomaly. |
|
|
Term
| How is an amniotic fluid index obtained? What are normal values? |
|
Definition
| Phelan et al. developed a method for evaluating and quantifying amniotic fluid volume at different intervals during a pregnancy. They divided the uterine cavity into four equal quadrants by two imaginary lines running perpendicular to each other. The largest vertical pocket of amniotic fluid, excluding fetal limbs or umbilical cord loops, was measured. The sum of the four quadrants was determined and called the amniotic fluid index (AFI). Normal values were calculated for each gestational age (plus or minus 2 standard deviations). Normal is 8 to 22 cm; decreased is less than 5 cm; increased is greater than 22 cm. |
|
|
Term
| What are the five parameters used when performing a biophysical profile? How are the scores assigned? What are normal values? |
|
Definition
Biophysical profile (BPP):
• BPP: Assign a value of two points to each of the following:
o Fetal breathing movement (FBM): One episode for 30 seconds continuously during a 30-minute observation
o Gross body movement: At least three discrete body/limb movements
in 30 minutes, unprovoked; continuous movement for 30 minutes should
be counted as one movement
o Fetal tone: Active extension and flexion; at least one episode of limbs or trunk
o Fetal heart rate (FHR): Also known as the non-stress test (NST); at least
two episodes of FHR of greater than 15 beats per minute and at least 15 seconds duration in a 20-minute period
o Amniotic fluid index (AFI): One pocket of amniotic fluid at least
2 cm in two perpendicular planes; or AFI total fluid measures between 5 and 22 cm.
o Cardiac non-stress test (NST): The following conditions indicate a reactive, or normal, NST and score two points:
• Two fetal heart rate accelerations of fifteen beats per minute or more
• Accelerations last at least 15 seconds
• Gross fetal movements are noted over 20 minutes without late decelerations |
|
|
Term
| What are normal S/D values for the umbilical and maternal uterine arteries? |
|
Definition
| The S/D ratio measures peak systole to end-diastolic blood flow. Increased vascular resistance is reflected by an increased S/D ratio or pulsatility index. According to Schulman an S/D ratio over 3.0 in the umbilical artery after 30 weeks is considered abnormal and demonstrates increased resistance in the fetal circulation. The maternal uterine artery S/D ratio should be below 2. A ratio above 2.6 suggests increased vascular resistance and indicates a decreased maternal blood supply to the uterus. |
|
|
Term
| List the applications of 3-D and 4-D ultrasound |
|
Definition
| 3-D ultrasound has created a new understanding of anatomy and pathology and is currently finding many clinical applications for abdominal, breast, cardiac, fetal, gynecologic, vascular, and other clinical modalities. Newer advances in technology have made it possible for information to be acquired quickly during real-time imaging of anatomic structures over time. This acquisition is called four-dimensional (4-D) ultrasound. This new age of volume acquisition and display are sometimes called the active matrix arrays by the ultrasound manufacturers. |
|
|
Term
| Describe the key to acquiring a beautiful fetal face in the third trimester |
|
Definition
The key to acquiring a beautiful fetal face in the third trimester is determined by fetal position (location), amniotic fluid, position of the placenta, and a high-quality 2-D image of the fetal profile. This technique has proved to be very productive:
o Once the fetus is in a mid-sagittal plane (profile) for a 3-D acquisition, place the ROI box on the fetal profile with the render–start line (dotted green line) above the profile with a layer of amniotic fluid and decrease the size of the ROI box close to the fetal structures.
o Next make a 3-D sweep and use the X, Y, and Z controls to rotate the image. The Voluson 730MT has mix and TH low knobs above the keyboard that can adjust the image for better resolution. (The 3-D and 4-D ultrasound machines have come a long way in a short period of time. The acquisition and reconstruction time has been greatly reduced as well as the resolution of the image.)
o Early learning experience for the sonographer beginning 3-D and 4-D ultrasound is to try a sweep at 90 degrees to the area of interest and use the different controls to optimize the volume rendered image.
o 3-D ultrasound should be used to complement the conventional 2-D scan and provide the sonographer or sonologist a more confident diagnosis. |
|
|
Term
| What is the major function of the placenta? |
|
Definition
| The primary function of the placenta is to permit the exchange of oxygenated maternal blood (rich in oxygen and nutrients) with deoxygenated fetal blood. Maternal vessels coursing posterior to the placenta circulate blood into the placenta, whereas blood from the fetus reaches this point through the umbilical cord |
|
|
Term
| Describe what comprises the fetal membranes. |
|
Definition
| The chorion, amnion, yolk sac, and allantois constitute the embryonic or fetal membranes. These membranes develop from the zygote. Implantation of the blastocyst occurs 6 to 7 days after fertilization. Enlargement of trophoblasts helps to anchor the blastocysts to the endometrial lining. The placenta has two components, the fetal portion, developed from the chorion frondosum (chorionic plate), and the maternal portion, the decidua basalis, formed by the endometrial surface. |
|
|
Term
| Describe the decidual changes of the placenta. |
|
Definition
• Decidua basalis: The decidual reaction that occurs between the blastocyst and the myometrium
• Decidua capsularis: The decidual reaction occurring over the blastocyst closest to the endometrial cavity
• Decidua vera (parietalis): Reaction changes in the endometrium opposite the site of implantation |
|
|
Term
| What is the major functioning unit of the placenta? |
|
Definition
| The major functioning unit of the placenta is the chorionic villus. Within the chorionic villus are the intervillous spaces. The maternal blood enters the intervillous spaces. The cotyledons are cobblestone in appearance and are composed of several main stem villi and their branches. They are covered with a thin layer of the decidua basalis. |
|
|
Term
| Name the functions of the placenta. |
|
Definition
Functions of the placenta:
• Respiration: Oxygen in maternal blood diffuses across the placental membrane into fetal blood by diffusion. Carbon dioxide passes in the opposite direction. Placenta acts as “fetal lungs.”
• Nutrition: Water, inorganic salts, carbohydrates, fats, proteins, and vitamins pass from maternal blood through the placental membrane into fetal blood.
• Excretion: Waste products cross membrane from fetal blood and enter maternal blood; excreted by mother’s kidneys.
• Protection: Some microorganisms cross placental border.
• Storage: Carbohydrates, proteins, calcium, and iron are stored in placenta and released into fetal circulation.
• Hormonal production: Produced by syncytiotrophoblast of placenta; hCG,
estrogens, progesterone, etc |
|
|
Term
| When should the amnion fuse with the chorion? |
|
Definition
| Expansion of the amniotic cavity occurs with the production of amniotic fluid. By 16 weeks the amnion fuses with the chorion and can no longer be seen on ultrasound as two separate membranes. If the separation extends beyond 16 weeks, it may be associated with polyhydramnios or prior amniocentesis. Hemorrhage also may have this appearance. |
|
|
Term
| Describe the placenta grades. |
|
Definition
• Grade 0: Represents the earliest placental grade with a smooth, well-defined chorionic plate, homogeneous placental tissue, and a regular basal plate (without echogenic densities). Typical grade of a placenta less than 28 weeks of gestation.
• Grade 1: Characteristic undulation (indentation) of chorionic plate with spotlike densities dispersed throughout the placental tissue (calcium deposits) with a regular basal plate
• Grade 2: Indentations of the chorionic plate with linear comma-like densities extending from the chorionic plate into the placental substance but not reaching the basal plate. Linear echogenic densities are noted along the basal plate.
• Grade 3: Highest grade of the placenta. The comma-like densities reach the basal plate as the placental septae are deposited with calcium, which surrounds the placental lobes (cotyledons), resulting in complete circles of calcium. Echo-spared areas may be found in the center with highly echogenic basal echoes that may produce acoustic shadowing. |
|
|
Term
| Name the primary causes of placentomegaly. |
|
Definition
| Placentomegaly is an enlarged placenta weighing more than 600 g. On ultrasound the placenta thickness measures more than 5 cm. Maternal diabetes and Rh incompatibility are primary causes for placentomegaly. Other causes include: maternal anemia, alpha-thalassemia, fetal–maternal hemorrhage, chronic intrauterine infections, twin-to-twin transfusion syndrome, congenital neoplasms, and fetal malformations. |
|
|
Term
| Distinguish among placenta accreta, increta, and percreta |
|
Definition
Placenta accreta is the abnormal adherence of part or all of the placenta with partial or complete absence of the decidua basalis. Chorionic villi grow into the myometrium, and the placental villi are anchored to muscle fibers rather than the intervening decidual cells. Placenta accreta occurs in approximately 1 in 2500 deliveries.
Placenta increta is further extension of the placenta through the myometrium. Placenta percreta is penetration of the uterine serosa. These conditions result from the underdeveloped decidualization of the endometrium. |
|
|
Term
| Describe the earliest sonographic evidence of an intrauterine pregnancy, including the sonographic appearance, location, and biochemical laboratory values. |
|
Definition
| The earliest sonographic findings of an intrauterine pregnancy are thickening of the decidua. Sonographically, this appears as an echogenic, thickening filling of the fundal region of the endometrial cavity. It occurs at approximately 3 to 4 weeks’ gestation. At approximately 4 weeks menstrual age, a small hypoechoic area appears in the fundus or midportion of the uterus known as the double decidual sac sign. As the sac embeds further into the uterus, it is surrounded by an echogenic rim and is seen within the choriodecidual tissue. This is known as the chorionic or gestational sac. At 5 weeks the average internal diameter of the gestational sac, calculated as the mean of the anteroposterior diameter, transverse diameter, and longitudinal diameter, can provide an adequate estimation of menstrual age. A gestational sac should be seen within the uterine cavity when the beta human chorionic gonadotropin (BhCG) is above 500 mIU/ml (second international standard). This becomes especially important when evaluating a pregnancy for ectopic implantation. |
|
|
Term
| What is the function of the yolk sac? Describe its sonographic appearance and size. |
|
Definition
It provides early transfer of nutrients from the trophoblast to the embryo. It also aids in the early formation of the primitive gut and vitelline arteries and veins, and the production of the primordial germ cells.
When the gestational sac exceeds 8 mm in mean internal diameter, a yolk sac should be seen |
|
|
Term
Describe the anatomic landmarks used to obtain a biparietal diameter and the
technique used to accurately measure it. |
|
Definition
| Obtain BPD of the fetal head at the transverse level of the midbrain; falx, septum pellucidum, and thalamic nuclei. Make sure the head is symmetric and oval. Measure from the outer to inner margins of the skull. In the third trimester, the BPD is not as accurate in predicting fetal age; it may approach ± 3 to 3.5 weeks. |
|
|
Term
| What is a cephalic index and what is the range of normal? |
|
Definition
The two most frequently noted alterations in head shape are dolichocephaly and brachycephaly. In dolichocephaly the head is shortened in the transverse plane (BPD) and elongated in the anteroposterior plane (OFD). In brachycephaly the head is elongated in the transverse diameter (BPD) and shortened in the anteroposterior diameter (OFD). One can underestimate gestational age from a dolichocephalic head or overestimate with brachycephaly. Because of these variations in fetal head shape, a cephalic index (CI) has been devised to determine the normality of the fetal head shape: CI = BPD/OFD/100.
A normal cephalic index is 80% |m1 standard deviation. The range of normal is 75% to 85%. A CI of greater than 85% suggests brachycephaly, and of less than 75% suggests dolichocephaly. |
|
|
Term
| Discuss the various orbital measurements and how they are obtained. |
|
Definition
| The ocular distance (OD), binocular distance (BD), and interocular distance (IOD) can be measured. The fetal orbit should be measured in a plane slightly more caudal than the BPD. The orbits are accessible in every head position except the occipitoanterior position (i.e., face looking down). All measurements should be taken from outer border to outer border. The OD measures a single fetal orbit. The BD includes both fetal orbits at the same time, whereas the IOD measures the length between the two orbits. |
|
|
Term
| Describe the two techniques used to measure a fetal cerebellum. What is the classic shape and why is the shape important? |
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Definition
| The cerebellum can be measured from the same level that the BPD is obtained, angling back into the posterior fossa to include the full length of the cerebellum. The cerebellum should have a dumbbell shape. The widest diameter of the cerebellum should be measured. The banana and lemon signs associate an abnormally shaped cerebellum with fetal spina bifida. In the presence of spina bifida, the fetal cerebellum is pulled downward into the foramen magnum, altering its shape to appear oblong or banana shaped. The frontal bones of the fetal skull also give in to this pressure, collapsing and giving the fetal head a lemon shape. |
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Term
| Describe the embryologic development of the umbilical cord. |
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Definition
| The umbilical cord forms during the first 5 weeks of gestation (7 menstrual weeks) as a fusion of the omphalomesenteric (yolk stalk) and allantoic ducts. The umbilical cord acquires its epithelial lining as a result of the enlargement of the amniotic cavity and the envelopment of the cord by the amniotic membrane. |
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Term
| The intestines are seen to herniate into umbilical cord on ultrasound between what period in development? |
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Definition
| The intestines grow at a faster rate than the abdomen and herniate into the proximal umbilical cord at approximately 7 weeks and remain there until approximately 10 weeks The insertion of the umbilical cord into the ventral abdominal wall is an important sonographic anatomic landmark because scrutiny of this area reveals abdominal wall defects such as omphalocele, gastroschisis, or limb–body wall complex. |
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Term
| What is the umbilical cord covered by? |
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Definition
| The umbilical cord is covered by the amniotic membrane. |
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Term
| The two umbilical arteries arise from which vessels? |
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Definition
| The umbilical artery arises from the fetal internal iliac vessels, courses alongside the fetal bladder, and exits the umbilicus to form part of the umbilical cord. The left umbilical vein enters the umbilicus and joins the left portal vein as it courses through the liver. The right umbilical vein usually regresses at 6 weeks’ gestation and is not seen by ultrasound. Persistence of the right umbilical vein is rare and may be related to an involution of the left umbilical vein. If it persists, the right umbilical vein enters the right lobe of the liver to join the right portal vein. At least 50% of these cases have other fetal anomalies. |
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Term
| With what conditions may a short umbilical cord be associated? |
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Definition
| A short umbilical cord measures less than 35 cm in length. This condition is associated with or predisposed to: oligohydramnios, restricted space (as in multiple gestations), intrinsic fetal anomaly, tethering of the fetus by an amniotic band, inadequate fetal descent, cord compression, and fetal distress. |
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Term
| What is the significance of a prominent umbilical cord diameter? |
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Definition
| The umbilical vein diameter increases throughout gestation, reaching a maximum diameter of 0.9 cm by 30 weeks’ gestation. The umbilical cord has been found to be significantly larger in fetuses of mothers with gestational diabetes than in the normal population, and the main increase in the width is attributed to an increase in Wharton’s jelly content. |
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Term
| Describe a marginal insertion of the cord. |
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Definition
| The differential proliferation of placenta villi may result in eccentric insertion of the umbilical cord into the placenta. The cord implants into the edge of the placenta instead of into the middle of the placenta. This is significant when the cord is inserted near the internal os as labor may cause the cord to prolapse or be compressed during contractions. The marginal insertion occurs in 2% to 10% of singleton births, 20% of twins, and 18% of pregnancies with a single umbilical artery. |
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Term
| How does an omphalomesenteric cyst form? |
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Definition
| The omphalomesenteric cyst is a cystic lesion of the umbilical cord caused by persistence and dilatation of a segment of the omphalomesenteric duct lined by epithelium of gastrointestinal origin. During the third week of early development, the omphalomesenteric duct joins the embryonic gut and yolk sac. This is closed by the 16th week of gestation; however, in some cases small vestigial remnants of duct may be found in normal umbilical cords. The omphalomesenteric cyst is found closer to the fetal cord insertion and may vary in size. This condition affects females over males with a ratio of 5:3. In addition, there may be an associated condition of Meckel’s diverticulum. |
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Term
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Definition
| Vasa previa is defined as the presence of umbilical cord vessels crossing the internal os of the cervix. The mortality may be high, ranging from 60% to 70% for vaginal delivery and is caused from rupture of the vessels and fetal exsanguination. Color Doppler is the best method of detection in the ultrasound examination. Vasa previa may result from many factors: velamentous insertion of the cord, succenturiate lobe of the placenta, or low-lying placenta with marginal insertion of the cord near the internal os. |
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Term
| The absence of cord twisting is an indirect sign of what condition? |
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Definition
| Coiling of the umbilical cord is normal and is related to fetal activity. The normal cord may coil as many as 40 times, usually to the left. Finberg states that the helical twisting of the cord can be easily determined by gross pathologic inspection. With the cord held vertically, vessels along the anterior surface that spiral downward from high left to low right, angled like the left side of the letter V indicate a left helix. The incidence of a left twist of the cord is found in 7:l pregnancies. The significance of this is that a fetus with a right twist in the cord has a higher incidence of fetal anomalies than with a left twist. The absence of cord twisting is an indirect sign of decreased fetal movement. This event occurs in a small number of deliveries; however, it may lead to increased mortality and morbidity. |
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Term
| What is the risk of a velamentous insertion of the cord? |
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Definition
| Velamentous insertion occurs when the cord inserts into the membranes before it enters the placenta rather than directly into the placenta. This condition occurs in 1% of singleton births, 12% of twins, and 9% of pregnancies with a single umbilical artery. There is an increased risk of thrombosis, cord rupture during delivery, or vasa previa. |
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Term
| What conditions may predispose to cord presentation and prolapse? |
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Definition
| Prolapse of the umbilical cord occurs when the cord lies below the presenting part. This condition may exist whenever the presenting part does not fit closely and fails to fill the pelvic inlet; further risk is incurred if the membranes rupture early. Compression of the cord reduces or cuts off the blood supply to the fetus and may result in fetal demise. Abnormal fetal presentation occurs in nearly half of the prolapse cord cases. A slightly higher risk is incurred when the fetus is in a transverse or breech presentation. Conditions predisposing to cord presentation and prolapse include: fetal abnormal presentation, nonengagement of the fetus because of prematurity, long umbilical cord, abnormal bony pelvic inlet, leiomyomas, polyhydramnios, vasa previa, velamentous insertion of the cord, marginal insertion of the cord in a low-lying placenta, and incompetent cervix with premature rupture of the membranes. |
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Term
| What is the incidence of congenital anomalies in a fetus with a single umbilical artery? |
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Definition
| Reports have found SUA in 18% of pregnancies with marginal insertion of the cord and in 9% with membranous insertion of the cord. The probable cause is atrophy of one of the umbilical arteries in the early development stage. The left umbilical artery is absent a slightly higher percentage of time than the right. Single umbilical artery has been associated with: congenital anomalies in 20% to 50% of cases, increased incidence of intrauterine growth retardation (small placenta), increased perinatal mortality, and increased incidence of chromosomal abnormalities. |
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Term
| Why is the production of amniotic fluid critical to fetal development? |
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Definition
| Amniotic fluid plays a vital role in fetal growth and serves several important functions during intrauterine life. Amniotic fluid allows the fetus to move freely within the amniotic cavity while maintaining intrauterine temperature and protecting the developing fetus from injury. Abnormalities of the fluid may interfere with the normal fetal development and cause structural abnormalities or may represent an indirect sign of an underlying anomaly, such as neural tube defect or gastrointestinal disorder. |
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Term
| Describe the production of urine and pathways of amniotic fluid. |
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Definition
| The fetal production of urine and the ability to swallow begins at 8 to 11 weeks’ gestation and becomes the major pathways for amniotic fluid production and consumption after this time period. The fetus swallows amniotic fluid, which is absorbed by the digestive tract. The fetus also produces urine, which is passed into the surrounding amniotic fluid. Fetal urination into the amniotic sac accounts for nearly the total volume of amniotic fluid by the second half of pregnancy, so the quantity of fluid is directly related to kidney function. A fetus with malformed kidneys or renal agenesis produces little or no amniotic fluid. |
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Term
| Name the six functions of amniotic fluid. |
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Definition
• To act as a cushion to protect the fetus
• To allow embryonic and fetal movements
• To prevent adherence of the amnion to the embryo
• To allow symmetric growth
• To maintain a constant temperature
• To act as a reservoir to fetal metabolites before their excretion by the maternal system |
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Term
| What happens to the volume of amniotic fluid as the pregnancy progresses? |
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Definition
| The volume of amniotic fluid increases to the 34th to 38th week of gestation and then slowly diminishes. The sonographer must be aware of the relative differences in amniotic fluid volume throughout pregnancy. During the second and early third trimester of pregnancy, amniotic fluid appears to surround the fetus and should be readily apparent. From 20 to 30 weeks of gestation, amniotic fluid may appear somewhat generous, although this typically represents a normal amniotic fluid variant. By the end of pregnancy, amniotic fluid is scanty, and isolated fluid pockets may be the only visible areas of fluid. |
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Term
| Describe how to perform the subjective assessment of amniotic fluid. |
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Definition
Subjective observation of amniotic fluid volumes throughout pregnancy helps the sonographer determine the norm and extremes of amniotic fluid. The amniotic fluid index aids in estimating the amount of amniotic fluid present in the uterine cavity. The amount of fluid volume correlates with fetal and placental weight. The small for age fetus has decreased amniotic fluid, whereas the large for age fetus has increased volume of fluid.
The subjective assessment is performed as the sonographer initially scans the entire uterus to determine the visual eyeball assessment of the fluid present, the lie of the fetus, and the position of the placenta. |
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Term
| What is the four-quadrant assessment? |
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Definition
| The four-quadrant assessment divides the uterine cavity into four equal quadrants by two imaginary lines running perpendicular to each other. The largest vertical pocket of amniotic fluid, excluding fetal limbs or umbilical cord loops, was measured. The sum of the four quadrants was determined and called the amniotic fluid index (AFI). Normal values were calculated for each gestational age (1/2 2 standard deviations). Normal is 8 to 22 cm; decreased is less than 5 cm; increased is greater than 22 cm. |
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Term
| Describe the single pocket assessment for amniotic fluid. |
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Definition
| Clinicians have described other criteria to define the presence of abnormal fluid volumes. Halpern has defined oligohydramnios as occurring when the maximum vertical pocket is less than 3 cm, whereas Manning et al. report less than 1 cm to be true oligohydramnios. Still other labs use 2 cm as the bottom line number. |
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Term
| What is the definition of polyhydramnios? |
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Definition
| Polyhydramnios is too much amniotic fluid. |
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Term
| What abnormal conditions is polyhydramnios associated with? |
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Definition
| Polyhydramnios is often associated with central nervous system disorders, which cause depressed swallowing. With gastrointestinal abnormalities often a blockage (atresia) of the esophagus, stomach, duodenum, and small bowel results in ineffective swallowing. Fetal hydrops, skeletal anomalies, and some renal disorders (usually unilateral) may be associated with hydramnios. Many congenital anomalies are found in association with polyhydramnios. |
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Term
| Describe how to assess if the fetus has oligohydramnios. |
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Definition
| Oligohydramnios is an overall reduction in the amount of amniotic fluid. The association between IUGR and decreased amniotic fluid (oligohydramnios) is well recognized. Oligohydramnios also has been associated with fetal renal anomalies, rupture of the intrauterine membranes, and the postdate pregnancy. If the intrauterine membranes are not ruptured and oligohydramnios is present before 28 weeks’ gestation, careful evaluation of the fetal renal system should be made by the sonographer to rule out renal agenesis, infantile polycystic disease, or posterior urethral valve syndrome. If the oligohydramnios is severe in a fetus with posterior urethral valves, the prognosis is not good. The identification of the renal area may be difficult to assess in the presence of severe oligohydramnios, and the use of color Doppler to demarcate the renal arteries may be helpful for the sonographer to determine if the kidneys are present or not. |
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Term
| What abnormal conditions may be associated with oligohydramnios? |
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Definition
Placental insufficiency may be a cause for the intrauterine growth retardation associated with oligohydramnios. The placental insufficiency produces a redistribution of fetal blood flow away from the kidneys and toward the brain to counterattack the hypoxia. This results in decreased urine output, which decreases the fluid volume. Cord compression by the fetus is another potential cause for fetal asphyxia leading to oligohydramnios. Often the sonographer may see signs of mild cord compression throughout the routine scan, with cardiac decompensation, or changes in Doppler flow patterns. Slight rotations of the mother during the examination may stimulate the fetus to roll away from the cord and thus blood is restored. If this cord compression becomes a chronic problem, the well-being of the fetus may be affected.
Renal disease or other clinical pregnancy problems may lead to diminished amounts of amniotic fluid. Premature rupture of the membranes may occur during the second trimester, so correlation with obstetric history is imperative. Nonanomalous conditions, such as intrauterine growth retardation and postdatism (beyond 42 weeks), are also causes of oligohydramnios. |
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Term
| What is amniotic band syndrome? |
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Definition
Amniotic band syndrome may represent a milder form of limb–body wall complex and may be predicted by amniotic bands (fibrous tissue strands) that entangle or amputate fetal parts.
Facial clefts, asymmetric encephaloceles, constriction or amputations defects of the extremities, and clubfoot deformities are common findings. |
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Term
| Differentiation between amniotic sheets and amniotic band syndrome. |
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Definition
| The sonographer may observe these bands as the real-time obstetric study is performed to observe where the band is attached to the uterine wall and what, if any, constriction is placed on the fetus. The careful observation with real-time allows the sonographer to observe if the fetus is free from the band or if the movement is restricted. Uterine sheets (synechiae) should not be confused with amniotic bands. |
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