Imperforate anus (IA) is a relatively common gastrointestinal anomaly, usually requiring prompt assessment and treatment after birth. Moreover, >50% of IA cases are associated with other congenital or chromosomal abnormalities. Therefore, the prenatal diagnosis of IA is considered ideal for optimizing neonatal treatment. Imperforate anus had rarely been diagnosed prenatally because of the absence of effective screening and diagnostic methods. Although the prenatal diagnosis of IA remains uncommon, cases of IA diagnosed prenatally have been increasing due to recent advances in the ultrasonographic evaluation of the anorectal anatomical structure of fetuses. A specific sonographic sign, absent or abnormal appearance of the anal sphincter muscles and anal canal mucosa in the tangential view of the fetal perineum, allows for the prenatal diagnosis of IA. High- and intermediate-type IAs are increasingly detected prenatally by this sign, while the prenatal diagnosis of low-type IA remains challenging. Several new sonographic techniques showing the precise anorectal anatomy, such as the sagittal view of the fetal pelvis, transvaginal approach, high-frequency linear transducer, and three-dimensional ultrasound, seem promising to further improve the diagnostic accuracy.
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