Ater delivery, the fundus of the contracted uterus lies slightly below the umbilicus. It consists mostly of myometrium covered by serosa and internally lined by decidua. The markedly attenuated lower uterine segment contracts and retracts, but not as forceully 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 cervical os. Immediately postpartum, 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.
Myometrial involution is a truly remarkable feat of destruction or deconstruction that begins as soon as 2 days ater delivery (Williams, 1931). The total number of myocytes does not decrease appreciably-rather, their size decreases markedly. As emphasized by Hytten (1995), the quality of studies that describe the degree of decreasing uterine weight postpartum are (8) cervices.
12.8 12.4 10 9 Cavity length 7.6 6.7 AP diameter 6.4
E) 10 12 ;:) 8 6
FIGURE 36-2 Sonographic measurements of uterine involution during the first 9 days postpartum. AP = anteroposterior. (Data from Hytten F: The Clinical Physiology of the Puerperium. London, Farrand Press, 1995.) poor. Best estimates show that the uterus weighs approximately 500 g by 1 week postpartum, about 300 g by 2 weeks, and at 4 weeks, involution is complete and the uterus weighs approximately 100 g. Ater each successive delivery, the uterus is usually slightly larger than before the most recent pregnancy.
Uterine involution and rapid dissipation of size progresses in the first week (Fig. 36-2). Sonographically, the uterus and endometrium return to pregravid size by 8 weeks postpartum (Bae, 2012; Steinkeler, 2012). In a study of 42 normal puerperas, Tekay and Jouppila (1993) identiied fluid in the endometrial cavity in 78 percent of women at 2 weeks, 52 percent at 3 weeks, 30 percent at 4 weeks, and 10 percent at 5 weeks. Belachew and coworkers (2012) used three-dimensional sonography and visualized intracavitary tissue matter in a third on day 1, in 95 percent on day 7, in 87 percent on day 14, and in 28 percent on day 28. By day 56, the small cavity was empty. Sohn and associates (l9��) described Doppler ultrasound results showing continuously increasing uterine artery vascular resistance during the first 5 days postpartum. Weintraub and colleagues (2013) posited that uterine involution may be diferent in preeclamptic women because they more likely had early diastolic notches seen on uterine artery velocimetry.
Because separation of the placenta and membranes involves the spongy layer, the decidua basalis is not sloughed. The in situ decidua varies markedly in thickness, it has an irregular jagged border, and it is infiltrated with blood, especially at the placental site. Within 2 or 3 days ater delivery, the remaining decidua becomes diferentiated into two layers. he 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.