The most frequently detected congenital digestive anomaly is isolated atresia [1, 4]. Usually, the first detected ultrasonographic findings are persistent polyhydramnios and dilatation of the digestive tract located proximal to the obstruction [5].

Esophageal atresia (EA) mostly occurs at the level of tracheal bifurcation. It is the most common type of digestive obstruction, accounting for approximately 35% of all cases, with a reported incidence of 1:2000–3000 live births. It appears as an isolated anomaly in approximately 60% of patients and as part of a syndrome or associated with other malformations in the remaining 40% [19, 20]. Prenatal US diagnosis accounts for 22%−31.7% of all cases [20,21,22]. Suggestive imaging findings include severe and persistent polyhydramnios, detected in approximately 56% of cases, variable distension of the hypopharynx, a dilated proximal esophagus or “cervical pouch sign”, and a persistent absent or small-sized stomach (50%) [13, 23] (Fig. 3). When performed following a suspicious US, MRI has good diagnostic accuracy, with a sensitivity of 94.7% and a specificity of 89.3% [22]. Midline sagittal dynamic cine-mode T2-W sequences during fetal swallowing may improve cervical pouch visualization and predict gap length [24].

In approximately 85% of patients with EA there is an associated tracheo-esophageal fistula (TEF) [20, 22]. Its antenatal diagnosis remains challenging, as a distal fistula can allow passage of fluid and modify the previously described imaging findings, rendering them absent or less evident. In patients with TEF, the amniotic fluid may be normal, and both the esophageal pouch and distended hypopharynx are less likely to be observed. The size of the stomach and the visualization of the distal esophagus are the key findings indicative of this diagnosis. However, although fetal MRI helps to improve diagnostic accuracy, the rate of prenatal detection remains low [22, 25] (Fig. 3).

Prenatal imaging findings of esophageal atresia (EA) Case 1 (a, b) EA with associated TEF: sagittal midline cervical plane (a) shows a distended proximal esophagus (white arrow, a). The black arrow (a) shows the thoracic aorta. Transverse US image of the upper abdomen (b) reveals an anomalous small stomach (arrow, b). Case 2 (c), pure EA. Midline sagittal T2-W image MR at 32GW reveals the esophageal pouch in the posterior mediastinum (arrow, c), filled with homogeneous hyperintense fluid, and absent stomach

The incidence of duodenal atresia ranges from 1–3:10000 live births, with approximately 60% comprising of isolated cases, and 40% associated with other congenital anomalies. These are mainly cardiac, genito-urinary, and chromosomal, like Down’s syndrome [17]. The European registry of congenital anomalies working group (EUROCAT) reports a prenatal US detection rate of 52–54% [20, 21]. Suggestive imaging findings include severe, persistent polyhydramnios and the classic “double bubble sign”, which graphically describes the distension of the fluid-filled stomach and the proximal duodenum (Fig. 4). However, this sign may be a false-positive and represent a transient finding in an otherwise healthy fetus [26]. In the case of inconclusive ultrasonographic findings, MRI can confirm the diagnosis and help to identify concomitant congenital anomalies, including multiple levels of atresia.

Prenatal imaging findings of duodenal atresia Transverse (a) screening US image at 23GW and transverse (b) and coronal (c) T2-W MR images at 30 GW show the classic “double bubble sign” with distended stomach (S) and proximal duodenum (D). Note increased amniotic fluid quantity in (b) and collapsed left bowel loops (arrow, c)

Small intestine atresia (jejunal or ileal) has an incidence range of 1:3000–5000 births and represents approximately 32% of all atresia [20]. Associated anomalies, also largely gastrointestinal, are observed in approximately 23% of cases [4, 22, 27]. The main imaging findings include persistent polyhydramnios and fluid-filled distension of the bowel loops located proximal to the atretic segment (Fig. 5) [28,29,30,31]. The prenatal detection rate is highly variable, ranging from 25 to 90% [4, 20]. Real-time ultrasonographic evaluation may identify hyperperistalsis of the distended bowel proximal to the obstruction, but accurate identification of the level and length of the obstruction is not always possible [31, 32]. Indeed, one of the main indications for MRI is the identification of the location of the atresia based on the number of dilated loops, the signal intensity characteristics of the fluid filling these loops, the size of colon and rectum, and the quantity and signal characteristics of meconium. These findings become more evident in the third trimester of pregnancy.

Imaging findings of proximal small bowel atresia with secondary volvulus. Transverse image (a) of screening US at 25GW, coronal T2- (b) and sagittal T1- (c) and T2-W (d) MR images at 30GW show distended stomach (S), duodenum (D) and proximal jejunal loops (J), filled with fluid isointense to the amniotic fluid. Note collapsed distal bowel loops, but normal-sized rectum, and normal intensity of distal meconium (arrow, c). Pregnancy was interrupted due to increased bowel dilatation and reduced peristalsis

A proximal jejunal atresia shows homogenous T1-hypointense and T2-hyperintense fluid in the dilated bowel, and an almost normal-sized colon and rectum, filled with normal meconium, produced by the intestinal secretions distal to the atresia (Fig. 5). A mid-small bowel obstruction displays a higher number of distended loops, filled with intermediate T1- and T2-W signal, and delayed detection and reduced quantity of distal meconium. Finally, a distal ileal atresia features multiple dilated loops, some of which are filled with meconium-like fluid signal, and delayed detection as well as scarce quantity of colorectal meconium. Reduced colon and rectum diameter or “microcolon” are also associated [9, 33]. An isolated jejunal or ileal atresia has an excellent long-term outcome after neonatal repair, but its association with the most important differential diagnosis - cystic fibrosis- considerably increases the risk of fetal morbidity and mortality [4].

Imaging findings of anorectal malformation. Axial US (a) and T2-W MR (b) images of the fetal perineum at 28GW show absent anal “target sign” (arrow). Midline sagittal T1-W (c) and T2-W (d) images reveal normal signal of the meconium in the transverse colon (arrow, c) but abnormal signal in the rectum (short arrows, c and d), suggesting recto-urinary communication, confirmed at birth. Urinary ascites and prominent hydrocele (H) were also noted in this fetus with associated renal anomalies (not shown) (B: Bladder)

Colonic atresia is commonly located proximal to the splenic flexure and accounts for about 5–15% of bowel atresia. Association with other congenital anomalies, mostly also gastrointestinal, is observed in approximately 30% of cases. It is difficult to identify prenatally, as the amniotic fluid index often remains normal, and proximal small bowel dilatation may be absent [18]. Anorectal malformation (ARM) is a complex spectrum of abnormalities, from imperforate anus to persistent cloaca. It can appear as an isolated congenital anomaly, but it is most often associated with other malformations, which are detected in approximately 70% of cases. They include VACTERL association, caudal regression syndrome and chromosomal abnormalities [34, 35]. The prenatal detection rate of anal atresia remains low - about 16% - and the diagnosis in utero is difficult, especially when isolated [3, 34]. An absent perineal “target sign” is the most important direct sign, followed by a distended, thick walled and high-located rectum, but visualization of the “target sign” cannot completely exclude low anal atresia [36]. Abnormal echogenicity and signal intensity of the rectal meconium suggests a communication between the rectum and the genito-urinary tract with secondary mixture of fluids. The presence of a fistula reduces the incidence of polyhydramnios, which if present, can be due to other associated anomalies (Fig. 6) [36].

Cloaca is the most severe form of ARM in females. In this pathology, the distal genital, urinary, and digestive tracts finish in a single common channel with a single external drainage [18]. Imaging findings include absent perineal “target sign”, a variable distended, fluid-filled vagina (hydrocolpos), and an often-displaced, high-located rectum with anomalous meconium signal. Homogenization of the fluids filling the different organs confirms their communication (Fig. 7) [2, 3, 29, 35].

In fetuses with atresia, MRI provides multiplanar imaging and allows differentiation between normal and abnormal meconium. These features help determine the level of obstruction, clarify complex anatomy, and identify associated congenital or multisystemic anomalies, particularly in syndromic cases [18]. MRI also improves the detection of fistulas [2, 3, 36].

Imaging findings of confirmed cloaca. Transverse (a), sagittal T2-W (b) and sagittal T1-W (c) images of a female fetus reveal identical signal intensity of the fluids filling the bladder (B) and the distended vagina (V), suggesting communicating organs. Note the displaced position of the rectum (arrows, b, c). Perineal-oriented T2-W image (d) shows absent anal “target sign” (arrow)

Multiple levels of digestive tract obstruction increase the risk of fetal and neonatal complications, postoperative complications, and perinatal mortality [4]. The jejunum is the most frequent site of multiple-level bowel atresia. Multifocal obstructions modify the typical imaging findings previously described. Sonographic real-time evaluation may identify hyperperistalsis of the distended bowel and changes in the gain help to differentiate between the fluids filling the intestinal loops [31, 32]. However, a complete prenatal diagnosis on US remains challenging and limits an accurate diagnosis of the levels of obstruction despite technical advances. MRI can show different signal intensities on T1- and T2-W sequences of the fluids located at different levels, with low T1- bowel fluid evident in cases of upper jejunal atresia and more intermediate T1- and T2- W signal in distally located obstructions [4] (Fig. 8). MRI can confirm a suspected ultrasonographic diagnosis, increase the detection rate for concomitant atresia, and help to identify the levels of obstruction [4, 14, 37].

Duplication cysts are benign lesions that result from abnormal gastrointestinal tract recanalization, with a reported incidence of approximately 1:4500-10.000 births [38]. Associated digestive anomalies are observed in approximately 10% of cases and non-digestive in approximately 16–26% [39]. Duplication cysts can develop at any point along the digestive system, with the ileum being the most frequently observed location (33%), followed by the esophagus (20%) and colon (13%) [32]. Histological examination confirms the diagnosis by identifying a cyst that shares a common wall with the gastrointestinal tract, contains a of smooth muscle layer, and has an internal epithelial lining [38].

Multiple levels of atresia. In case 1 (a, b), MRI exam was performed at 30GW after US diagnosis of polyhydramnios and duodenal atresia. Coronal T2-W image (a) shows a classical “double-bubble sign” with a distended stomach (S) and duodenum (D). The midline sagittal T2-W image (b) reveals a “cervical pouch sign” (arrow), not previously detected. In case 2, a coronal T2-W MR image (c) at 34 GW shows distended intestinal loops filled with fluids with different signal intensities, indicating multiple levels of obstruction

Prenatal US identifies the pathology in only about 20–30% of cases [40]. US examination typically features an elongated, tubular or spherical, mostly unilocular cyst, in direct contact with the digestive wall, and with a thick and well-defined multilayered wall [41]. The classic “double wall sign” is considered the most important finding for distinguishing duplication cysts from other more frequently observed fetal cysts (such as the ovarian cyst in females), but it is not always present on US [42]. MRI may reveal the direct anatomical relationship between the cyst and the gastrointestinal tract, which helps to confirm the diagnosis in inconclusive cases (Fig. 9) [41, 43]. Communication with the intestinal lumen remains difficult to clarify in utero but, in our experience, fluid with meconium-like signal intensity characteristics filling colonic or rectal located cysts suggests communication.

Imaging findings in rectal duplication cyst. Axial US image (a) at 25GW shows a pre-sacral located, well-defined cystic lesion in the fetal pelvis (arrow), with a thick wall, but provides limited value in evaluation of the adjacent organs. Midline sagittal T1- (b) and T2-WI (c) images of MRI at 30GW confirm the thick wall of the cyst (arrows) and show its direct anatomical relationship with the rectum (R), filled with normal meconium, (B: Bladder). The absence of contact with the coccyx excludes sacrococcygeal teratoma