During pregnancy, numerous dramatic physiologic changes take place. After delivery, whether by the vaginal or cesarean route, these changes resolve even more rapidly. This process of resolution of the pelvic organs occurs during the puerperium, which is the 6 weeks following delivery. In 1972, Robinson first described the use of diagnostic ultrasound to image the postpartum pelvic organs.¹ Since that time, ultrasound has continued to play an important role in the diagnosis of postpartum anatomical structures, particularly in conditions of infection, hemorrhage, or suspected retained placenta; processes that require imaging include assessment of women with persistent postpartum endomyometritis, pelvic abscesses, retained placental fragments, and hematomas of the uterus, bladder, peritoneal flap, or abdominal wall. Such complications can involve up to 5% to 10% of postpartum women.²

This chapter reviews normal and pathologic sonographic findings during the puerperium, with the major emphasis on the pelvic organs, although the urinary tract, hepatobiliary tract, and vasculature are also covered. The normal anatomy, including variations of normal, and the changes associated with pregnancy and further alterations by the various disorders are reviewed.

Normal Anatomy

The Uterus

In the nonpregnant state, the uterus usually occupies a midline position that measures approximately 8 cm in length. Uterine size may be affected by parity and pathologic conditions such as leiomyomata or adenomyosis.³ The uterus grows from a pregravid weight of 140 g to a peak of 1 kg at term; the uterine blood flow increases from 50 mL/min to 500 mL/min at term.² The uterus shrinks remarkably during the puerperium, without cellular destruction; instead, there is a reduction in cell size with a simultaneous loss and resorption of tissue fluid and contractile proteins. Similarly, the animal model shows an orderly process of cellular restitution with cytoplasmic and collagen disintegration without tissue necrosis. An awareness of the expected uterine size is essential to the interpretation of the ultrasound findings (Figure 31-1). The most rapid proportion of uterine involution occurs during the first 2 weeks postpartum with a nearly 50% reduction of uterine size; these findings are noted on traditional 2D and 3D sonography.^4-7 Wachsberg et al⁶ have demonstrated that the postpartum uterus approaches the nongravid state within 6 to 8 weeks following delivery. These investigators also noted that uterine contractions and bladder size may affect the measurements. Route of delivery does not affect the postpartum uterine dimensions.

The postpartum uterus involutes rapidly:⁴

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  At 24 hours: L 17.5 cm × W 12.3 cm × anteroposterior (AP) 9.0 cm; endometrial AP cavity = 1.2 cm

  
  
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  1 week: L 12.9 cm × W 11.3 cm × AP 8.7 cm; endometrial AP cavity = 1.3 cm

  
  
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  2 weeks: L 11.0 cm × W 7.7 cm × AP 7.8 cm; endometrial AP cavity = 1.0 cm

The postpartum myometrium shows a heterogeneous echo appearance related to the pregnancy-related changes: increased vascularity, and resolution of edema and fluid content. The vascularity usually resolves relatively quickly, such that by 3 to 4 weeks postdelivery (see Figure 31-2), the vascular channels should have the pregravid appearance.⁶ Uterine fibroids may continue to disrupt the usually quiescent appearance of the heterogeneity.

The endometrial cavity maintains a consistent measurement of less than 2 cm in the anteroposterior dimension during the early puerperium. Variations of normal may be noted. For instance, a small amount of fluid may be seen in many normal postpartum uteri and should not be connoted to be pathologic. Furthermore, the endometrial–myometrial interface may be variable; it may be smooth and well defined or irregular and heterogeneous. One of the most clinically important roles of sonography of the postpartum uterus is to identify retained placenta or products of conception. An enlarged endometrial cavity (AP diameter >2.5 cm on transabdominal sonography) is often associated with a hypotonic uterus or retained products. It is often difficult to discern blood clots from placenta fragments; however, an echogenic mass within the uterine cavity in the face of secondary postpartum hemorrhage (PPH) is very strong evidence for retained products.^9,10 Deans and Dietz¹¹ studied 94 postpartum women prospectively and correlated normal uterine findings with the clinical course and subdivided their sonographic findings to the upper and lower uterus. The upper uterine segment area had an average thickness of 13.8 mm and an average volume of 35.6 cm³, and the lower segment/cervical area held considerably more material with an average volume of 54.8 cm³. The overall endometrial echo appearance will differ depending on the uterine size and postpartum state.

Vulva, Vagina, and Pararectal Space

The vagina is a hollow muscular tube that extends from the vulvar vestibule to the uterus. Whereas the lower third of the vagina is closely opposed to the urogenital diaphragm and pelvic diagram, the upper and middle thirds of the vagina are supported by the levator ani and the cardinal ligaments. The vagina undergoes marked distension during the delivery process. Lacerations to the vagina or damage to blood vessels adjacent to the vagina can occur. In women who have undergone an instrumented vaginal delivery, or those patients with a coagulation defect such as associated with placental abruption, vulvar or vaginal hematomas are not uncommon. Ultrasound may help to delineate a vaginal hematoma, but magnetic resonance imaging (MRI) is probably superior in assessing possible extension into the retroperitoneal space.¹¹ Also, in general, the clinical picture will dictate the management.

Similarly, hematomas of the vulva or distal vagina may extend into the paravaginal or pararectal space. In this manner, women may lose significant blood into these potential spaces without external manifestation of hemorrhage or outward hematoma. Imaging with computed tomography (CT) or MR is superior to ultrasound for assessment of these conditions due to the nature of the anatomical space and the tenderness of genital tract hematomas.¹¹

The Broad Ligament

The broad ligaments are double reflections of the parietal peritoneum extending from the bony pelvic side walls to the uterus. They envelope the uterine vasculature, the fallopian tubes, and to some extent, the ovaries. In the normal patient, the broad ligament is not easily visualized sonographically. Nevertheless, pathological conditions within the broad ligament such as hematoma, abscess, fluid collections, or fibroids may be easily seen (Figure 31-3).

The Ovaries

In the nonpregnant woman, the ovaries are usually found near the ovarian fossa, which are depressions of the peritoneum adjacent to the external iliac vessels near the pelvic brim. During the puerperium, the ovaries are lifted by the utero-ovarian ligaments above the true pelvis and can be demonstrated sonographically only about half the time (Figure 31-4).⁴ During the first trimester, ovarian cysts such as corpus lutea are often seen; after this time, these cysts usually regress. Not infrequently, luteomas of pregnancy are seen, usually noted as multicystic ovaries, often bilateral, with thin septations. Luteomas are physiologic changes of the ovaries in response to the human chorionic gonadotropic (hCG) hormone and may lead to the production of androgens. Hence, luteomas of pregnancy may be seen in patients with conditions of markedly elevated hCG levels such as with molar pregnancies or multiple gestations. These changes in the ovaries almost invariably resolve with resolution of the pregnancy, although they can persist for months; unwarranted surgery may lead to profuse hemorrhage. Their bilateral thin-walled appearance is typical. Rarely, maternal or female fetal virilization may be seen.¹²

The Cul-de-sac

The cul-de-sac is the space between the uterus and the rectum and is the most dependent portion of the pelvis. Ascitic fluid or blood may settle in this region. In the absence of free fluid, the cul-de-sac is difficult to visualize sonographically. A small amount of fluid or blood following delivery is normally present.

Various extraperitoneal spaces are usually not seen sono­graphically. These compressed potential cavities include the retropubic space of Retzius, and pararectal, paravesical, and paravaginal spaces where blood or fluid may possibly accumulate. Operative vaginal delivery, maternal coagulopathy, cesarean delivery, and rarely spontaneous hemorrhage may lead to hematomas in these regions (Figure 31-5). In conditions of suspected retroperitoneal pathology, CT or MR imaging is usually superior to ultrasound.¹³

Pathology

Postpartum Hemorrhage

PPH is typically divided into early (<24 hours after delivery) and late (>24 hours). Early PPH is usually due to uterine atony, genital tract lacerations, coagulopathy, or retained placental fragments. Often, clinical circumstances such as a boggy uterus hints toward the etiology in early PPH, and the management is directed without ultrasound. However, the concomitant use of ultrasound can be useful as an adjunct to curettage of the boggy uterus because the usual palpable definition of the uterine cavity may be absent to the operator. The postpartum uterus is easy to perforate.²

Late PPH, which is much less common than early PPH, is seen in less than 1% of postpartum women and usually is related to either subinvolution of the placental implantation site or retained placental tissue (Figure 31-6). One of the most difficult diagnostic challenges to the sonologist is distinguishing retained placental fragments from uterine blood clots. Blood clots tend to be homogeneous, whereas placental fragments tend to be more echogenic; however, this is not universal. This is an important differentiation, in that retained placental fragments left in situ can lead to further hemorrhage or infection; in contrast, postpartum curettage will increase the likelihood of damage to the decidual basalis layer leading to uterine synechiae and infertility.¹⁵ Sonography has been shown to be helpful in identifying products of conception after spontaneous abortion.¹⁵ Dewhurst¹⁶ demonstrated that less than one-third of 89 patients who underwent dilatation and curettage (D&C) for PPH had pathologically confirmed choronic villi. Placenta accreta can be a contributing factor to retained products.^17,18

Hertzberg and Bowie²¹ suggested five categories of diagnosis when assessing the puerperal uterus for retained products with ultrasound (see Figure 31-6). In their series of 53 patients, the finding of an echogenic mass was the most common finding associated with retained placenta. When a thin endometrial stripe or only endometrial fluid was seen, they found no patients with retained products on follow-up pathological examination or clinical follow-up. Their five categories include¹⁹: