Acute appendicitis is the most common non-obstetric surgical condition in pregnancy [4, 5]. Early and accurate diagnosis is of paramount importance because of increased fetal loss rate of 30 % seen with appendiceal rupture compared to <2 % fetal loss rate in the absence of rupture [6]. The clinical diagnosis of appendicitis can be difficult due to atypical location of pain secondary to cephalad migration of the cecum and appendix by the gravid uterus [2], as well as physiologic leukocytosis observed during pregnancy. MRI can be useful to exclude appendicitis if a normal appendix is visualized (Fig. 18.3). MRI has been shown to be superior in the visualization of the normal appendix compared to ultrasound [7], and the appendix can be visualized with high accuracy in the pregnant patient, estimated at 83–91 % [8–10]. In the acute setting, MRI shows good diagnostic accuracy of appendicitis with sensitivity of 80–100 % and specificity of 93.6–100 % [7, 9].

The MR imaging findings of acute appendicitis are similar to that of the nonpregnant patient. T2-weighted imaging demonstrates a dilated appendix and mural thickening (Fig. 18.4). Periappendiceal fluid can be seen as high signal intensity on T2-weighted images and may be more readily visualized utilizing fat suppression techniques. Periappendiceal abscess can be seen in the setting of a ruptured appendix (Fig. 18.5). Contrast is rarely needed for diagnosis; however, if contrast is administered, the appendix would demonstrate mural enhancement.

Inflammatory bowel disease (IBD) has a peak age distribution similar to that of childbearing age in the female, with peak incidence at 15–25 years [11]. IBD is a common mimicker of appendicitis in that the terminal ileum is involved in approximately 80 % of patients with IBD [11, 12]. Patients will typically present with diffuse abdominal pain, diarrhea, nausea, and vomiting. The colon can be involved with associated involvement of the small intestine in approximately 50 % of cases and in isolation in about 15–20 % of cases [11]. MR is accurate in the evaluation of IBD and its complications including abscess, fistula, and strictures [11, 12].

Typical imaging features include concentric wall thickening, bowel wall, and mesenteric edema presenting as high T2 signal intensity in the acute phase [13, 14]. There may be associated mesenteric lymphadenopathy or fibrofatty proliferation, as well as reactive free fluid [13, 14].

The majority of bowel obstructions during pregnancy are secondary to adhesions, estimated at up to 58 % [15]. Additional causes include volvulus, intussusception, and inflammatory bowel disease. Imaging features consist of dilated fluid-filled bowel loops of small and/or large bowel. Small bowel is considered dilated when measured greater than 2.5 cm in diameter [16] and large bowel when measured greater than 6 cm or greater than 9 cm at the cecum [17]. Bowel obstruction is best depicted utilizing HASTE/SSFSE or FISP/FIESTA images (Fig. 18.6).

The presence of an enlarged, gravid uterus and displacement of intra-abdominal structures can cause small bowel or omentum to protrude through an existing or acquired ventral or umbilical hernia. This can lead to bowel obstruction or omental ischemia and pain from incarceration (Fig. 18.7). Signs suggestive of incarceration include fluid within the hernia sac, wall thickening of herniated bowel loops, and dilatation of bowel proximal to the hernia [18].

Acute cholecystitis is the second most common condition necessitating surgery during pregnancy [12]. Ninety percent of cases of acute cholecystitis during pregnancy are secondary to gallstones [15]. Pregnancy is a known risk factor for cholelithiasis as the pregnant state leads to increased cholesterol synthesis by estrogen and decreased gallbladder motility by progesterone [19]. Ultrasound is the primary method for diagnosis, but MR boasts relatively high sensitivity of 88 % and specificity of 89 % [15]. Imaging findings include gallbladder wall thickening and edema with pericholecystic fluid, best demonstrated as high signal intensity on T2-weighted images. While these findings are readily diagnosed on ultrasound, it can be very difficult to image choledocolithiasis with ultrasound alone. MRCP can detect stones within the biliary tree with a sensitivity of 89–100 % and specificity of 83–100 % [20]. Choledocolithiasis is depicted as a low signal intensity filling defect on T2-weighted/MRCP images with upstream dilatation above the site of the filling defect [20] (Fig. 18.8). One must be careful not to mistake flow artifact, which is seen as an intermediate signal intensity, centrally located filling defect on a single sequence, for a choledocolith [21]. MRCP can also limit the performance of unnecessary ERCP when no stones are visualized in the biliary system.