Physiology of the Puerperium

PHYSIOLOGY OF THE PUERPERIUM

Involution of the Uterus

The uterus weighs approximately 1000 g and has a volume of 5000 mL immediately after delivery, compared with its nonpregnant weight of approximately 70 g and capacity of 5 mL. Immediately after delivery, the fundus of the uterus is easily palpable halfway between the pubic symphysis and the umbilicus. The immediate reduction in uterine size is a result of delivery of the fetus, placenta, and amniotic fluid, as well as the loss of hormonal stimulation. Further uterine involution is caused by autolysis of intracellular myometrial protein, resulting in a decrease in cell size but not in cell number. As a result of these changes, the uterusreturns to the pelvis by 2 weeks postpartum and is at its normal size by 6 weeks postpartum. Immediately after birth, uterinehemostasis is maintained by contraction of the smooth muscle of the arterial walls and compression of the vascu-lature by the uterine musculature.

Lochia

As the myometrial fibers contract, the blood clots from the uterus are expelled and the thrombi in the large vessels of the placental bed undergo organization. Within the first 3 days, the remaining decidua differentiates into a super-ficial layer, which becomes necrotic and sloughs, and a basal layer adjacent to the myometrium, which had con-tained the fundi of the endometrial glands. This basal layer is the source of the new endometrium.

The subsequent discharge, called lochia, is fairly heavy at first and rapidly decreases in amount over the first 2 to 3 days postpartum, although it may last for several weeks. Lochia isclassically described as: (1) lochia rubra, menses-like bleed-ing in the first several days, consisting mainly of blood and necrotic decidual tissue; (2) lochia serosa, a lighter dis-charge with considerably less blood in the next few days; and (3) lochia alba, a whitish discharge which may persist for several weeks and which may be misunderstood as ill-ness by some women, requiring explanation and reassur-ance. In women who breastfeed, the lochia seems to resolve more rapidly, possibly because of a more rapid involution of the uterus caused by uterine contractions associated with breastfeeding. In some patients, there is an increased amount of lochia 1 to 2 weeks after delivery, because the eschar that developed over the site of placental attachment has been sloughed. By the end of the third week post-partum, the endometrium is reestablished in most patients.

Cervix and Vagina

Within several hours of delivery, the cervix has reformed, and by 1 week, it usually admits only one finger (i.e., it is approximately 1 cm in diameter). The round shape of the nulliparous cervix is usually permanently replaced by a transverse, fish–mouth-shaped external os, the result of laceration during delivery. Vulvar and vaginal tissues returnto normal over the first several days, although the vaginal mu-cosa reflects a hypoestrogenic state if the woman breastfeeds, be-cause ovarian function is suppressed during breastfeeding. Themuscles of the pelvic floor gradually regain their tone.

Vaginal muscle tone may be strengthened by the use of Kegel exercises, consisting of repetitive contractions of these muscles.

Return of Ovarian Function

The average time to ovulation is 45 days in nonlactating women and 189 days in lactating women. Ovulation is sup-pressed in the lactating woman in association with elevated prolactin levels. In these women, prolactin remains elevated for 6 weeks, whereas in nonlactating women, prolactin levels return to normal by 3 weeks postpartum. Estrogen levels fall imme-diately after delivery in all patients, but begin to rise ap-proximately 2 weeks after delivery if breastfeeding is not initiated. The likelihood of ovulation increases as the frequencyand duration of breastfeeding decreases.

Abdominal Wall

Return of the elastic fibers of the skin and the stretched rec-tus muscles to normal configuration occurs slowly and is aided by exercise. The silvery striae gravidarum seen on the skin usually lighten in time. Diastasis recti, separation of the rectus muscles and fascia, also usually resolves over time.

Cardiovascular System

Pregnancy-related cardiovascular changes return to nor-mal 2 to 3 weeks after delivery. Immediately postpartum, plasma volume is reduced by approximately 1000 mL, caused primarily by blood loss at the time of delivery. During the immediate postpartum period, there is also a significant shift of extracellular fluid into the intravascular space. The increased cardiac output seen during preg-nancy also persists into the first several hours of the post-partum period. The elevated pulse rate that occurs during pregnancy persists for approximately 1 hour after delivery, but then decreases.

These cardiovascular events may contribute to the decompen-sation that sometimes occurs in the early postpartum period in patients with heart disease.

Immediately after delivery, approximately 5 kg of weight is lost as a result of diuresis and the loss of ex-travascular fluid. Further weight loss varies in rate and amount from patient to patient.

Hematopoietic System

The leukocytosis seen during labor persists into the early puerperium for several days, thus minimizing the usefulness of identifying early postpartum infection by laboratory evi-dence of a mild-to-moderate elevation in the white cell count.

There is some degree of autotransfusion of red cells to the intravascular space after delivery as the uterus contracts.

Renal System

Glomerular filtration rate represents renal function andremains elevated in the first few weeks postpartum, then returns to normal. Therefore, drugs with renal excretion should be given in increased doses during this time. Ureter and renal pelvis dilation regress by 6 to 8 weeks. There maybe considerable edema around the urethra after vaginal delivery, resulting in transitory urinary retention. About 7% of womenexperience urinary stress incontinence, which usually re-gresses by 3 months.