Ultrasound diagnosis of fourth branchial arch sinus at 22 weeks' gestation.

Fourth branchial arch sinus is the rarest of the branchial cleft anomalies and its prenatal detection has not previously been reported in the English literature. We describe a case of fourth branchial arch sinus identified in a fetus during a routine ultrasound evaluation at 22 weeks' gestation. Survey of the fetal anatomy revealed a small 'nipple-like' structure on the neck, arising anteriorly to the sternocleidomastoid muscle, midway between the chin and the left shoulder. The structure persisted unchanged throughout the pregnancy. The prenatal diagnosis of fourth branchial arch sinus was confirmed at delivery at term.