First-trimester rudimentary horn pregnancy: prerupture ultrasound diagnosis.

A case of pregnancy in the rudimentary horn of a bicornuate uterus is presented, and a discussion of this rare and potentially catastrophic phenomenon follows. This is the first such case reported in which the diagnosis was made early in gestation through the use of ultrasonography. An extrauterine gestation accompanied by a well-defined placenta visible on ultrasound scan is suggested as a criterion for differentiating rudimentary horn pregnancy from abdominal pregnancy.