Very large yolk sac and bicornuate uterus in a live birth.

Pregnancies with a very large yolk sac are generally always associated with poor outcomes. Herein, we present a case in which a very large yolk sac (mean diameter, 8.1 mm) and a bicornuate uterus were shown on transvaginal sonography in the first trimester, but which resulted in a live birth. To our knowledge, this is the largest yolk sac among all cases accompanying live births which have been published in the literature. The unusual enlargement of the yolk sac may be secondary to the compensation effect for the poor blood supply in the medial wall of the bicornuate uterus for the conceptus. To predict the outcome of pregnancy, based on this case report, the quality of the yolk sac might be more important than its size.