This process begins immediately after expulsion of the placenta with contraction of the uterine smooth muscle.

At the end of the third stage of labor, the uterus is in the midline, approximately 2 cm below the level of the umbilicus, with the fundus resting on the sacral promontory. At this time, the uterus weighs approximately 1000 g.

Within 12 hours, the fundus can rise to approximately 1 cm above the umbilicus. By 24 hours after birth, the uterus is about the same size as it was at 20 weeks of gestation. Involution progresses rapidly during the next few days. The fundus descends 1 to 2 cm every 24 hours. By the sixth postpartum day, the fundus is normally located halfway between the umbilicus and the symphysis pubis. The uterus should not be palpable abdominally after 2 weeks and should have returned to its nonpregnant location by 6 weeks after birth .

The uterus, which at full term weighs approximately 11 times its prepregnancy weight, involutes to approximately 500 g by 1 week after birth and to 350 g by 2 weeks after birth. At 6 weeks postpartum, it weighs 60 to 80 g. 

Increased estrogen and progesterone levels are responsible for stimulating the massive growth of the uterus during pregnancy. Prenatal uterine growth results from both hyperplasia (an increase in the number of muscle cells) and hypertrophy (an enlargement of the existing cells). After birth, the decrease in these hormones causes autolysis—the self-destruction of excess hypertrophied tissue. The additional cells laid down during pregnancy remain and account for the slight increase in uterine size after each pregnancy. Subinvolution is the failure of the uterus to return to a nonpregnant state. The most common causes of subinvolution are retained placental fragments and infection

Postbirth uterine discharge, initially bright red (lochia rubra) and may contain small clots. For the first 2 hours after birth, the amount of uterine discharge should be about that of a heavy menstrual period. After that time, the lochial flow should steadily decrease.

Lochia may continue for 2 to 6 weeks after the birth but may last longer and still be normal. Thus lochia persists up to 4 to 8 weeks after birth.

Recurrence of bleeding 7 to 14 days after birth is from the healing placental site.

If the woman receives an oxytocic medication, regardless of the route of administration, the flow of lochia is often scant until the effects of the medication wear off.

Lochial Bleeding

• Lochia usually trickles from the vaginal opening. The steady flow is greater as the uterus contracts.

• A gush of lochia may result as the uterus is massaged. If it is dark in color, it has been pooled in the relaxed vagina, and the amount soon lessens to a trickle of bright red lochia (in the early puerperium).

Nonlochial Bleeding

• If the bloody discharge spurts from the vagina, there can be cervical or vaginal tears in addition to the normal lochia.

• If the amount of bleeding continues to be excessive and bright red, a tear can be the source.

• Immediately after birth, the cervix is soft. 
• The ectocervix appears bruised and has some small lacerations, the risk of infection.
• Within 2 to 3 days postpartum becomes shortened, firmer, and regains form.
• Second or third day postpartum, cervix is dilated 2 to 3 cm, and by 1 week is 1 cm dilated.
• External cervical os never regains its prepregnancy appearance; no longer a circular shape. Appears more as a jagged slit often described as a "fish mouth".
• Lactation delays the production of cervical and other estrogen-influenced mucus and mucosal characteristics.

• Significant hormonal changes occur during the postpartal period. Expulsion of the placenta results in dramatic decreases of the hormones produced by that organ. Decreases in human chorionic somatomammotropin (also called human placental lactogen), estrogens, cortisol, and the placental enzyme insulinase reverse the diabetogenic effects of pregnancy, resulting in significantly lower blood glucose levels in the immediate puerperium.

Prolactin levels in blood rise progressively throughout pregnancy. After birth, as levels of estrogen and progesterone decrease, prolactin levels increase. In women who breastfeed, prolactin levels are highest during the first month after birth and remain elevated above nonpregnant levels as long as the woman is breastfeeding. Serum prolactin levels are influenced by the frequency of breastfeeding, the duration of each feeding, and the degree to which supplementary feedings are used. Individual differences in the strength of an infant’s sucking stimulus also affect prolactin levels. In nonlactating women, prolactin levels decline after birth and reach the prepregnant range by the third postpartum week.

Ovulation occurs as early as 27 days after birth in nonlactating women, with a mean time of about 7 to 9 weeks. About 70% of nonbreastfeeding women resume menstruating by 12 weeks after birth. The mean time to ovulation in women who breastfeed is about

6 months. The persistence of elevated serum prolactin levels in breastfeeding women appears to be responsible for suppressing ovulation. In lactating women, both the resumption of ovulation and the return of menses are determined in large part by breastfeeding patterns. For example, ovulation is delayed longer in women who breastfeed exclusively compared with women who breastfeed and offer supplemental infant formula to their infants.

Because of the uncertainty about the return of ovulation and menstruation, discussion of contraceptive options early in the puerperium is necessary. The first menstrual flow after childbirth is usually heavier than normal. Within three or four cycles, the amount of menstrual flow returns to the woman’s prepregnancy volume.

Colostrum or early milk, a clear, yellow fluid, may be expressed from the breasts.

(1) elimination of uteroplacental circulation reduces the size of the maternal vascular bed by 10% to 15%; (2) loss of placental endocrine function removes the stimulus for vasodilation;

(2) loss of placental endocrine function removes the stimulus for vasodilation.(3) mobilization of extravascular water stored during pregnancy

(3) mobilization of extravascular water stored during pregnancy occurs. By the third postpartum day, the plasma volume has been replenished as extravascular fluid returns to the intravascular space

Normal leukocytosis of pregnancy averages approximately 12,000/ mm3. During the first 10 to 12 days after childbirth, values between 20,000/mm3 and 25,000/mm3 are common.

Headache requires careful assessment. Postpartum headaches can be caused by various conditions, including postpartum-onset preeclampsia, stress, and leakage of cerebrospinal fluid into the extradural space during placement of the needle for administration of epidural or spinal anesthesia.