INVOLUTION OF THE REPRODUCTIVE TRACT
 Birth Canal
 Return to the nonpregnant state begins soon after delivery. The vagina and its outlet gradually diminish in size but rarely regain their nulliparous dimensions. Rugae begin to reappear by the third week but are less prominent than before. The hymen is represented by several small tags of tissue, which scar to form the myrtiform caruncles. Vaginal epithelium begins to proliferate by 4 to 6 weeks, usually coincidental with resumed ovarian estrogen production. Lacerations or stretching of the perineum during delivery may result in vaginal outlet relaxation. Some damage to the pelvic floor may be inevitable, and parturition predisposes to urinary incontinence and pelvic organ prolapse. This is discussed in detail in Chapter 27 (p. 536).
Uterus
 The massively increased uterine blood flow necessary to maintain pregnancy is made possible by significant hypertrophy and remodeling of pelvic vessels. After delivery, their caliber gradually diminishes to approximately that of of the prepregnant state. Within the puerperal uterus, larger blood vessels become obliterated by hyaline changes, are gradually resorbed, and are replaced by smaller ones. Minor vestiges of the larger vessels, however, may persist for years. During labor, the margin of the dilated cervix, which corresponds to the external os, may be lacerated. The cervical opening contracts slowly and for a few days immediately after labor, readily admits two fingers. By the end of the first week, this opening narrows, the cervix thickens, and the endocervical canal reforms. The external os does not completely resume its pregravid appearance. It remains somewhat wider, and typically, ectocervical depressions at the site of lacerations become permanent. These changes are characteristic of a parous cervix (Fig. 36-1). The markedly attenuated lower uterine segment contracts and retracts, but not as forcefully as the uterine corpus. During the next few weeks, the lower segment is converted from a clearly distinct substructure large enough to accommodate the fetal head to a barely discernible uterine isthmus located between the corpus and internal os. FIGURE 36-1 Common appearance of nulliparous (A) and parous (B) cervices. Cervical epithelium also undergoes considerable remodeling, and this actually may be salutary. Ahdoot and associates (1998) found that approximately half of women showed regression of high-grade dysplasia following vaginal delivery. Kaneshiro and coworkers (2005) found similar regression—about 60 percent overall—regardless of delivery mode. Postpartum, the fundus of the contracted uterus lies slightly below the umbilicus. It consists mostly of myometrium covered by serosa and internally lined by basal decidua. The anterior and posterior walls, which lie in close apposition, are each 4 to 5 cm thick (Buhimschi, 2003). At this time, the uterus weighs approximately 1000 g. Because blood vessels are compressed by the contracted myometrium, the uterus on section appears ischemic compared with the reddish-purple hyperemic pregnant organ. Myometrial involution is a truly remarkable tour de force of destruction or deconstruction that begins as soon as 2 days after delivery as shown in Figure 36-2. As emphasized by Hytten (1995), studies that describe the degree of decreasing uterine weight postpartum are poor quality. Best estimates are that by 1 week, the uterus weighs approximately 500 g; by 2 weeks, about 300 g; and at 4 weeks, involution is complete and the uterus weighs approximately 100 g. After each successive delivery, the uterus is usually slightly larger than before the most recent pregnancy. The total number of myocytes does not decrease appreciably—rather, their size decreases markedly. FIGURE 36-2 Cross sections of uteri made at the level of the involuting placental site at varying times after delivery. p.p. = postpartum. (Redrawn from Williams, 1931.) Sonographic Findings Uterine size dissipates rapidly in the first week (Fig. 36-3). And the uterus and endometrium return to pregravid size by 8 weeks after delivery (Belachew, 2012; Steinkeler, 2012). In a study of 42 normal women postpartum, Tekay and Jouppila (1993) identified fluid in the endometrial cavity in 78 percent at 2 weeks, 52 percent at 3 weeks, 30 percent at 4 weeks, and 10 percent at 5 weeks. Demonstrable uterine cavity contents are seen for up to 2 months following delivery. Belachew and colleagues (2012) used 3-dimensional sonography and visualized intracavitary tissue matter in a third on day 1, 95 percent on day 7, 87 percent on day 14, and 28 percent on day 28. By day 56, the small cavity was empty. Sohn and associates (1988) described Doppler ultrasound results showing continuously increasing uterine artery vascular resistance during the first 5 days postpartum.
Decidua and Endometrial Regeneration Because separation of the placenta and membranes involves the spongy layer, the decidua basalis is not sloughed. The remaining decidua has striking variations in thickness, it has an irregular jagged appearance, and it is infiltrated with blood, especially at the placental site (see Fig. 36-2). Within 2 or 3 days after delivery, the remaining decidua becomes differentiated into two layers. The superficial layer becomes necrotic and is sloughed in the lochia. The basal layer adjacent to the myometrium remains intact and is the source of new endometrium. This arises from proliferation of the endometrial glandular remnants and the stroma of the interglandular connective tissue. Endometrial regeneration is rapid, except at the placental site. Within a week or so, the free surface becomes covered by epithelium, and Sharman (1953) identified fully restored endometrium in all biopsy specimens obtained from the 16th day onward. Histological endometritis is part of the normal reparative process. Moreover, microscopic inflammatory changes characteristic of acute salpingitis are seen in almost half of women between 5 and 15 days, but these findings do not reflect infection (Andrews, 1951). Clinical Aspects Afterpains. In primiparous women, the uterus tends to remain tonically contracted following delivery. In multiparas, however, it often contracts vigorously at intervals and gives rise to afterpains, which are similar to but milder than labor contractions. These are more pronounced as parity increases and worsen when the infant suckles, likely because of oxytocin release (Holdcroft, 2003). Usually, afterpains decrease in intensity and become mild by the third day. We have encountered unusually severe and persistent afterpains in women with postpartum uterine infections. Lochia. Early in the puerperium, sloughing of decidual tissue results in a vaginal discharge of variable quantity. The discharge is termed lochia and contains erythrocytes, shredded decidua, epithelial cells, and bacteria. For the first few days after delivery, there is blood sufficient to color it red—lochia rubra. After 3 or 4 days, lochia becomes progressively pale in color—lochia serosa. After approximately the 10th day, because of an admixture of leukocytes and reduced fluid content, lochia assumes a white or yellow-white color—lochia alba. The average duration of lochial discharge ranges from 24 to 36 days (Fletcher, 2012).