Pelvic ultrasonography is an adjunct to the bimanual pelvic examination for many postpartum women presenting to the emergency department with pelvic pain or vaginal bleeding. Table 1 shows the common causes of postpartum hemorrhage (PPH) and pain.

PPH is described as blood loss exceeding 500 mL after vaginal delivery or 1,000 mL after a cesarean section, or any amount of blood loss that results in maternal hemodynamic compromise. PPH can be divided into two groups: early (primary) PPH refers to bleeding that occurs within 24 hours after delivery, and late (secondary) PPH takes place between 24 hours and 6 to 12 weeks after delivery [2].

The most common cause of primary PPH is uterine atony, which is most often a clinical diagnosis, although ultrasonography may be utilized in order to exclude other etiologies [3]. Secondary PPH is less common than primary PPH, but can cause significant maternal morbidity and mortality [4]. Secondary PPH is frequently a sign of underlying RPOC or endometritis.

Acute pelvic pain and vaginal bleeding are two of the most common presenting symptoms of postpartum patients in the emergency room. Making the correct diagnosis may be challenging in these cases due to the lack of specific laboratory tests and sometimes equivocal physical examination findings. Therefore, imaging is usually required to aid the differential diagnosis. Ultrasonography is the primary imaging modality for assessing pelvic pathology in the postpartum period. In addition to its excellent diagnostic utility, ultrasonography is ideal because of its wide availability, portability, and lack of ionizing radiation.

Pelvic ultrasonography can be performed via a transabdominal or transvaginal approach. The transvaginal examination is performed with a high-frequency transvaginal transducer (>7 MHz). It provides excellent near-field resolution, allowing for a detailed evaluation of the endometrial cavity. The transabdominal evaluation is performed with a convex low-frequency transducer (1-5 MHz). It provides a wider field of view that can visualize free fluid or blood products outside the lower pelvis. It may also provide a better view of the ovaries in patients where the uterus remains enlarged in the early postpartum period.

The normal postpartum uterus has a variable sonographic appearance. The endometrial cavity may contain anechoic fluid or echogenic material consistent with blood products (Fig. 1) [5,6]. These findings can be seen in both symptomatic and asymptomatic patients [7]. Therefore, the clinical context is essential for interpreting the significance of these findings.

The term RPOC refers to residual fetal, placental, or other decidual tissue remaining within the uterus after delivery, miscarriage, or pregnancy termination [8]. The risk of RPOC is higher when termination occurs after the start of the second trimester.

RPOC show a variable appearance on grayscale ultrasonography, ranging from a hyperechoic or mixed-echogenicity mass in the endometrial cavity to a heterogeneously thickened endometrial stripe with varying degrees of vascularity on color Doppler ultrasonography. An irregular interface between the endometrium and myometrium is another grayscale finding that has also been reported in the setting of RPOC [6,9]. If the endometrial echo-complex (EEC) is less than 10 mm without an endometrial mass, RPOC are extremely unlikely.

Color Doppler ultrasonography can enhance diagnostic accuracy in identifying RPOC. In patients in whom RPOC is suspected based on the clinical circumstances, the presence of vascularity in a thickened EEC or endometrial mass is highly suggestive of RPOC. The degree of vascularity can be divided into four groups by comparing endometrial vascularity to the adjacent normal myometrium (Figs. 2-6) [8,10]. Since RPOC may be avascular, the absence of color Doppler flow does not exclude the diagnosis. Avascular RPOC (type 0) and intrauterine blood clots demonstrate no flow on color Doppler ultrasonography images [10]. However, if the echogenic material within the endometrial cavity appears to be free-floating, this is more suggestive of a blood clot rather than RPOC.

When evaluating possible type 3 RPOC, it is essential to differentiate it from an arteriovenous malformation (AVM). This can be challenging, but as a general rule, the hypervascularity of type 3 RPOC will be centered in the endometrium, whereas the vascularity of uterine AVMs (UAVMs) should be centered in the myometrium [10]. UAVMs can be a life-threatening condition that requires endovascular treatment. Therefore, in cases of suspected RPOC with markedly increased vascularity, potentially either type 3 RPOC or AVM, this information should be conveyed clearly to the obstetrician to avoid dilation and curettage (D&C).

Endometritis is an infection of the endometrium and can occur after delivery or any intervention in the uterus. The risk of endometritis is greater following cesarean section than vaginal delivery [11]. The risk factors for endometritis include RPOC, prolonged labor, and premature rupture of membranes [12]. The symptoms of endometritis may include fever, pelvic pain, uterine tenderness, purulent vaginal discharge, and uterine subinvolution (prolonged postpartum enlargement of the uterus).

Endometritis is a clinical diagnosis, and its imaging findings may overlap with expected normal postpartum changes. The infected endometrium can demonstrate normal postpartum sonographic features without any specific imaging findings of infection. Fluid and echogenic debris within the endometrial cavity can be consistent with hemorrhage or clot and does not always indicate infection [6]. Alternatively, the endometrium may have a thickened and heterogeneous appearance with increased vascularity (Fig. 7). As usual, the clinical scenario will contribute greatly to the interpretation of these findings. Air within the endometrial cavity may be seen. On ultrasonography, this will manifest as echogenic foci with dirty shadowing or the ring-down artifact, which raises suspicion for infection (Fig. 7). It should be noted, however, that endometrial gas can be a normal finding for up to 3 weeks after delivery and may be seen in up to 20% of healthy postpartum patients [13]. Endometritis is treated with intravenous broad-spectrum antibiotics. If an infected hematoma, RPOC, or uterine abscess is detected, curettage may be required. Severe complications of untreated endometritis such as peritonitis, phlegmon, abscess, ovarian vein thrombosis, and septic thrombophlebitis occur in 1% to 4% of patients [11].

UAVMs occur when there is a direct communication between the arterial and venous systems without an intervening capillary bed [14]. UAVMs are a rare cause of postpartum bleeding and may be congenital or acquired.

Ultrasonography is the initial imaging method of choice to evaluate patients with UAVMs. Grayscale findings are often variable and non-specific, including myometrial heterogeneity or thickening. Sometimes an ill-defined heterogeneous intramural or endometrial mass may have been seen, containing multiple cystic or tubular structures (Fig. 8) [15]. Color and spectral Doppler examinations are essential to demonstrate the vascular nature of the lesion with flow in the aforementioned cystic tubular structure [14]. Doppler interrogation will show multidirectional flow with aliasing and a mosaic pattern associated with turbulent high-velocity flow [14]. If a UAVM is suspected in a patient, imaging studies should be performed prior to intrauterine instrumentation in order to exclude this diagnosis, as D&C could lead to life-threatening hemorrhage.

Uterine artery pseudoaneurysm (UAP) is an abnormal outpouching of an artery that results after arterial injury [16]. Pseudoaneurysm occurs when leakage of arterial blood into surrounding tissue results in persistent communication between the artery and the contained collection of blood.

Grayscale ultrasonography images demonstrate a cystic structure within the uterus, and the color Doppler assessment should show bidirectional swirling arterial flow, which may resemble the yin-yang symbol (yin-yang sign) (Figs. 9, 10). During spectral analysis, a "to-and-fro" waveform should be seen at the neck of the pseudoaneurysm, and blood passes into and out of the pseudoaneurysm [16]. UAPs are at risk of rupture or expansion; hence an urgent intervention is required. Endovascular treatment or embolization is the first treatment choice for UAPs.

Ovarian vein thrombosis (OVT) is a rare complication that occurs in 0.02% to 0.18% of pregnancies [17]. Most (80%-90%) cases occur in the right ovarian vein due to the usual dextrotorsion of the enlarged uterus during pregnancy, which causes compression of the right ovarian vein against the pelvic rim [17]. Patients may be asymptomatic or may complain of vague abdominal pain and fever. OVT may resolve spontaneously, but it can also lead to severe complications such as inferior vena cava (IVC) thrombosis, pulmonary embolism, and sepsis. Grayscale and color Doppler ultrasonography images may demonstrate tubular or serpiginous hypoechoic structures in the adnexa adjacent to the ovarian artery without demonstrable color Doppler flow (Fig. 11) [12]. Extension of the thrombus into the IVC may be visible with ultrasonography, as well as enlargement of the ovary on the affected side.

Bladder flap hematomas occur following cesarean delivery due to inadequate hemostasis and manifest as a hematoma between the bladder wall and the lower uterine segment. On ultrasound images, a bladder flap hematoma will be seen as a mixed-echogenicity avascular mass anterior to the lower segment of the uterus and posterior to the bladder wall (Fig. 12) [6]. A small bladder flap hematoma (<2 cm) is a common complication after cesarean delivery (Fig. 13). A large bladder flap hematoma (>5 cm) should alert the radiologist to the possible uterine dehiscence [18].

A subfascial hematoma results from injury of the epigastric vessels or their branches in the abdominal wall, and consists of a hematoma in the prevesical space, also known as the space of Retzius (Figs. 12, 14). On ultrasonography images, a cystic or complex mass will be seen deep to the rectus muscles and anterior to the bladder (Fig. 14). Significant blood loss may occur from a subfascial hematoma because the prevesical space can accommodate fluid up to a volume of 2,500 mL [19]. The presence of gas or internal septations is concerning for infected hematoma or abscess, both for subfascial and bladder flap hematomas (Fig. 14).

Uterine rupture is a rare but serious complication that usually presents with abdominal pain and may cause significant hemoperitoneum. Most uterine ruptures occur in patients with a history of previous cesarean delivery, although uterine rupture may also occur spontaneously or due to trauma. Grayscale ultrasonographic findings include a defect in the myometrium, a protruding amniotic sac (in a gravid patient), an extrauterine hematoma, a bulky empty uterus, and hemoperitoneum [20].

Uterine dehiscence (or cesarean incisional dehiscence) is characterized by a myometrial defect with an intact overlying serosal layer. The diagnosis of uterine dehiscence can be challenging due to significant overlap with the normal postoperative appearance of a cesarean delivery incision. The presence of a large bladder flap hematoma (>5 cm) has been associated with uterine rupture [18]. Contrast-enhanced pelvic MRI can be helpful to demonstrate an intact serosal layer with its robust soft tissue contrast resolution [6].