Sonographic depiction of the fetal anus and its utility in the diagnosis of anorectal malformations.

OBJECTIVES: The purpose of this study was to describe the sonographic appearance of the fetal anus and its usefulness in diagnosis of anal atresia. METHODS: An anomaly scan was performed in 13,150 patients over 8 years. Gestational ages ranged from 16 to 38 weeks. A tangential scan of the fetal perineum was performed. The anus was seen as a hypoechoic ring representing the wall of the anal canal with a central echogenic dot representing the lumen. This appearance was seen posterior to the external genitalia. If there was failure to see this appearance, a coronal scan of the fetal pelvis was done to look for the anal canal in contiguity with the rectum and to confirm its extension up to perineum. Failure to see these two features was considered diagnostic of anal atresia. RESULTS: On the basis of these criteria, anal atresia was diagnosed in 17 fetuses. Sixteen of these fetuses also had other associated anomalies. The appearance of the anus was useful for ruling out anal atresia in 2 fetuses with a dilated colon containing echogenic meconium balls. CONCLUSIONS: The anus is visible on a tangential scan of the fetal perineum. It can be seen routinely during an anomaly scan to diagnose or rule out anal atresia.