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| Fertilization occurs,usually occurring in the ampulla of the uterine tube. |
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| Implantation is complete by the end of the second week after fertilization. The most growth occurs in the outer cells, or trophoblast, which eventually becomes the fetal part of the placenta. The inner cell mass that will develop into the baby becomes flattened into the embryonic disk. Cells that eventually form part of the fetal membranes develop. |
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Many women miss their first menstrual period during the third week after conception. The embryonic disk develops three layers called germ layers that in turn give rise to major organ systems of the body (Table 6-3). The three germ layers are the ectoderm, mesoderm, and endoderm.
The central nervous system begins developing during the third week. A thickened flat neural plate appears, extending toward the end of the embryonic disk that will become the head. The neural plate develops a longitudinal groove that folds to form the neural tube. At the end of the third week the neural tube is fused in the middle but still open at each end. |
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The shape of the embryo changes during the fourth week after conception. It folds at the head and tail end and laterally, resembling a C-shaped cylinder. A “tail” is apparent during the embryonic period because the brain and spinal cord develop more rapidly than other systems. The tail disappears as the rest of the body catches up with growth of the central nervous system. The neural tube closes during the fourth week. If the neural tube does not close, defects such as anencephaly and spina bifida result.
Formation of the face and upper respiratory tract begins. Beginnings of the internal ear and the eye are apparent. The upper extremities appear as buds on the lateral body walls. Because the embryo is sharply flexed anteriorly, the heart is near the embryo's mouth. Partitioning of the heart into four chambers begins during the fourth week and is completed by the end of the sixth week.
The lower respiratory tract begins growth as a branch of the upper digestive tract, which is a simple tube at this time. Gradually, the esophagus and trachea complete separation. The trachea branches to form the right and left bronchi. These bronchi in turn branch to form the three lobes of the right lung and two lobes of the left lung. Continued branching of the bronchi eventually forms the terminal air sacs, or alveoli. The alveoli proliferate and become surrounded by a rich capillary network that enables oxygen and carbon dioxide exchange at birth. |
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| The head is very large because the brain grows rapidly during the fifth week after fertilization. The heart is beating and developing four chambers. Upper limb buds are paddle shaped with obvious notches between the fingers. Lower limbs form slightly later than upper ones. Lower limbs are also paddle shaped, but the area between the toes is not as well defined as the division between the fingers. |
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The rapidly developing head is bent over the chest. The heart reaches its final four-chambered form. Upper and lower extremities continue to become more defined.
The eyes continue to develop, and the beginnings of the external ears appear as six small bumps on both sides of the neck. Facial development begins with eyes, ears, and nasal pits that are widely separated and aligned with the body walls. Gradually the embryo grows so that the face comes together in the midline and the external ears assume their proper position on the sides of the head. |
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Definition
General growth and refinement of all systems occur seven weeks after conception. The face becomes more human looking. The eyelids begin to grow, and the extremities become longer and better defined. The trunk elongates and straightens, although a C-shaped spinal curve remains in the newborn at birth.
The intestines have been growing faster than the abdominal cavity during the embryonic period. The relatively large liver and kidneys also occupy much of the abdominal cavity. Therefore most of the intestines are contained within the umbilical cord while the abdominal cavity grows to accommodate them. The abdomen is large enough to contain all its normal contents by 10 weeks. |
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| The embryo has a definite human form, and refinements to all systems continue. The ears are low set but approaching their final location. The eyes are pigmented but not yet fully covered by eyelids. Fingers and toes are stubby but well defined. The external genitalia begin to differentiate, but male and female characteristics are not distinct until 10 weeks after conception, or 12 weeks after the woman's last menstrual period. |
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The head is approximately half the total length of the fetus at the beginning of this period. The body begins growing faster than the head, changing the proportions. The extremities approach their final relative lengths, although the legs remain proportionately shorter than the arms. The first fetal movements begin but are too slight for the mother to detect.
The face is broad with a wide nose and widely spaced eyes. The eyes close at 9 weeks and reopen at 26 weeks. The ears appear low set because the mandible is still small.
The intestinal contents that were partly contained within the umbilical cord enter the abdomen by 11 weeks as the capacity of the abdominal cavity catches up with them in size. Blood formation occurs primarily in the liver during the ninth week but shifts to the spleen by the end of the twelfth week. The fetus begins producing urine during this period and excretes it into the amniotic fluid.
Internal differences in males and females begin to be apparent in the seventh week. External genitalia look similar until the end of the ninth week. By the end of the twelfth week, the fetal gender can be determined by the appearance of the external genitalia. |
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| The fetus grows rapidly in length, so the head becomes smaller in proportion to the total length. Movements strengthen, and some women, particularly those who have been pregnant before, are able to detect them. This phenomenon is referred to as quickening. The face looks human because the eyes face fully forward. The ears near their final position at the sides of the head and in line with the eyes. |
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Fetal movements feel like fluttering or “butterflies.” Some women may not recognize these subtle sensations.
Changes in the skin and hair are evident. Vernix caseosa, a fatty, cheeselike secretion of the fetal sebaceous glands, covers the skin to protect it from constant exposure to amniotic fluid. Lanugo is fine, downy hair that covers the fetal body and helps the vernix adhere to the skin. Both vernix and lanugo diminish as the fetus reaches term. Eyebrows and head hair appear.
Brown fat is a special heat-producing fat deposited during this period that helps the newborn maintain temperature stability after birth. It is located on the back of the neck, behind the sternum, and around the kidneys. |
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While continuing to grow and gain weight, the fetus still appears thin because of minimal subcutaneous fat. The skin is translucent and red because the capillaries are close to its fragile surface.
The lungs begin to produce surfactant, a surface-active lipid substance that facilitates lung expansion and makes it easier for the baby to breathe after birth. Surfactant reduces surface tension in the lung alveoli and prevents them from collapsing with each breath. Production of surfactant begins at approximately 20 weeks but does not reach levels that permit easy survival outside the uterus until 26 to 28 weeks after conception. Surfactant production increases during late pregnancy, particularly during the last 2 weeks (Moore & Persaud, 2003a). To reduce respiratory distress of prematurity, artificial surfactant may be given to infants who are at risk for a deficiency because of their immaturity. Maternal corticosteroids such as betamethasone may be given before birth to accelerate surfactant production in the preterm fetus likely to be born before completion of 34 weeks gestation (see Chapter 27 for discussion of preterm labor care).
The capillary network surrounding the alveoli is increasing but still very immature, although some gas exchange is possible. A fetus born at this gestation is less likely to survive, because of inadequate gas exchange. Other systems are extremely immature as well, such as blood vessels in the brain that may bleed. |
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The fetus is more likely to survive if born during this period because of maturation of the lungs, pulmonary capillaries, and central nervous system. The fetus becomes plumper with smoother skin as subcutaneous fat is deposited under the skin. The skin gradually becomes less red. The eyes, which were closed during the ninth week, reopen. Head hair is abundant. Blood formation shifts from the spleen to the bone marrow.
During early pregnancy the fetus floats freely within the amniotic sac. However, the fetus usually assumes a head-down position during this time for two reasons:
▪ The uterus is shaped like an inverted egg. The overall shape of the fetus in flexion is similar, with the head being the small pole of the egg shape and the buttocks, flexed legs, and feet being the larger pole.
▪ The fetal head is heavier than the feet, and gravity causes the head to drift downward in the pool of amniotic fluid.
The head-down position is also most favorable for normal birth. |
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| The skin is pigmented according to race and is smooth. Larger vessels are visible over the abdomen, but small capillaries cannot be seen. Toenails are present, and fingernails extend to the fingertips. The fetus has more subcutaneous fat, which rounds the body contours. If the fetus is born during this period, chances of survival are good. |
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Definition
Growth of all body systems continues until birth, but the rate of growth slows as full term approaches. The fetus is mainly gaining weight. The pulmonary system matures to enable efficient and unlabored breathing after birth.
The well-nourished term fetus is rotund with abundant subcutaneous fat. At birth, boys are slightly heavier than girls. The skin is pink to brownish pink, depending on race. Lanugo may be present over the forehead, upper back, and upper arms. Vernix often remains in major creases such as the groin and axillae.
The testes are in the scrotum. Breasts of both male and female infants are enlarged, and breast tissue is palpable beneath the areola and nipple. |
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