The scan revealed a slighter increased NT (3.2 mm), but no other soft markers for aneuploidy. At the lower abdominal level, mostly in the left quadrant, we noticed an anechoic oblong structure with well-demarked smooth walls, without a Doppler signal containing a pediculated hyperechogenic image that moved inside the anechoic one (Figures 1, 2, 3 and 4). The complex image was labeled as distal bowel dilatation after checking the complete separation from the neighboring structures (urinary bladder). Fetal sex was confirmed as male, and a thorough evaluation of the fetal anatomy was performed. No other anomalies were found. Anorectal atresia diagnosis was advanced. Considering the possible association with chromosomal anomalies, free fetal DNA test was recommended and it was repeated because of the inconclusive first result. The second result indicated a high risk (6/100) for trisomy 21. Amniocentesis with MCA was performed with a normal result (arr (1-22)x2,(XY)x1).  Close follow-up of the fetus was conducted monthly. At the mid-trimester anomaly scan, the anechoic image disappeared, and a hyperechoic rimmed structure of 13 mm was found instead (Figure 5). No other anomalies were obvious. Later in pregnancy, even this image vanished. Considering the initial diagnosis, we evaluated the perianal muscular complex (PAMC). The examination was difficult due to the fetal position, but at 29 weeks the PAMC aspect was abnormal and the typical “target sign” was not obvious (Figure 6). From this consideration, we counseled the couple as having a very high likelihood of ARA, and pediatric surgery evaluation was scheduled.

At 36 weeks, the baby was delivered by caesarean section, with 2770 g and a 9 Apgar score. Postnatal examination revealed anorectal atresia with a fistula at the perineal level, allowing meconium passage (Figures 7 and 8). The newborn underwent surgical evaluation with dilatation of the fistula and was scheduled for surgery at 3 months of life.

Discussion

Anal atresia occurs as an isolated congenital anomaly, with a reported frequency of 1.11 per 10,000 births in Europe⁽¹⁾. Anal atresia is more often associated with other congenital anomalies, such as VACTERL syndrome (vertebral defects, anal atresia, tracheoesophageal fistula, esophageal atresia, renal anomalies and limb malformations) or caudal regression syndrome^(9-11). Prenatal diagnosis is based on the presence of a dilated fetal bowel on ultrasound⁽¹²⁾. This diagnosis is commonly corroborated by the presence of further congenital anomalies since anal atresia rarely occurs as an isolated anomaly⁽⁹⁾. In a cohort study of patients with ultrasound consistent anal atresia, Harris et al. confirmed the diagnosis either at birth or at fetal autopsy. Among them, 11 of 12 patients (92%) with anal atresia had abnormalities associated with either VACTREL syndrome or caudal regression syndrome⁽¹³⁾. In our case, the abnormality was isolated.

It is well known that anorectal anomaly is more frequent in male fetuses⁽¹⁴⁾, and in our case the fetus was male.

The etiology of ARA appears to be multifactorial. Chromosomal aberrations have been reported in only one case of Down syndrome. Genetic mutations involved in this condition are rare⁽¹⁵⁾. In our case, it was assumed that the abnormality was related to the assisted reproductive procedure.

The diagnosis of ARA may be difficult in the first trimester when the diagnosis is based only on indirect ultrasound signs. ARA has a low prenatal detection rate of only 16%⁽²⁾. In a 10-year study on 56 children, prenatal diagnosis of ARA by MRI versus ultrasound could not be established below 20 weeks of gestation⁽¹⁶⁾.

Fourteen cases have been reported in the literature in which the diagnosis was suspected following visualization of fetal intraabdominal or pelvic cystic structures^(17-20). Cystic structures are represented by distended intestines filled with fluid of undetermined origin. The accumulation of fluid can be explained by the fact that the intestinal mucosa is not capable of absorption in early gestation and the rectum ends in the fundus⁽²¹⁾.

The most common form of these structures is tubular, which is determined by associated anomalies (rectovesical fistula, megacystis, complex cloacal anomalies). A tubular structure was also the defining feature in our case.

The presence of intracystic hyperechogenic structures may suggest the existence of intestinal disorders due to enterolithiasis⁽⁷⁾. In our case, the hyperechogenic structure was interpreted as an image produced by peristalsis.

The pregnancy assessment and management of a cystic abdominal tubular formation in a fetus at 12 weeks of gestation should take into consideration the differential diagnosis⁽²²⁾, including the possibility that bowel dilatation is a transient finding⁽²³⁾. Cystic transient images in pregnancy with spontaneous resolution in the second and third trimesters are reported in the literature, particularly if the right colon is affected⁽²¹⁾. In our case, the anechogenic image faded in the second trimester. It is thus recommended that serial sonographic examinations and perhaps other tests, including karyotyping, be offered to try to exclude other significant malformations. According to a review by Pleş et al., the appearance of abnormalities on later examination was reported not only as transient findings, but also as a change in echogenicity from anechoic to hyperechoic structures in two cases⁽⁸⁾.

Conclusions

The diagnosis of anorectal atresia in the prenatal period is clinically important because prenatal diagnosis allows the planning of birth and the postnatal management. This case demonstrates the importance of maintaining a high clinical suspicion for anal atresia in a fetus with intraabdominal tubular anechogenic imaging in the first trimester and monitoring with serial ultrasound to establish a diagnosis of certainty. 

Conflict of interests: The authors declare no con­flict of interests.