Visualization of the fetal anus by prenatal ultrasound for the diagnosis of anorectal malformations: is it feasible?

The goal of this study was to determine the feasibility of identifying the anal dimple (AD) on routine prenatal ultrasound. Using the presence, absence, appearance, and location of the anal dimple as an indirect sign for possible underlying anorectal malformations (ARM), we hypothesize that evaluation of the anal dimple as part of the fetal anatomic survey may increase the sensitivity in detecting less severe ARMs. In a prospective longitudinal observational study, pregnant women who underwent prenatal ultrasound (US) at the Colorado Fetal Care Center between January 2019 and 2020 were enrolled. The variables recorded included gestational age, singleton versus multiple pregnancy, gender of the fetus, visualization of the AD, and reason for non-visualization of the AD. A total of 900 ultrasounds were performed, evaluating 1044 fetuses, in 372 different pregnant women. Gestational ages ranged from 16 to 38 weeks. The AD was visualized in 612 fetuses (58.6%) and not seen in 432 (41.4%). The two most common reasons for non-visualization were extremes in gestational age (n = 155; 36%) and fetal position (n = 152; 35.3%). The optimal gestational age range for AD visualization was 28-33 weeks + 6 days, with 78.1% visualization rate. Visualization of the anal dimple by ultrasound is feasible and may aid in the detection of less severe ARMs, ultimately impacting pregnancy management and family counseling. The optimal timing for anal dimple visualization is late second and third trimester.