Disappearance of one gestational sac in the first trimester of multiple pregnancies--ultrasonographic findings.

Three patients with twin pregnancy documented ultrasonographically in the first trimester, who then lost one sac spontaneously yet continued with a live single fetus, are reported. From the literature, it appears that the incidence of twin pregnancy in the first trimester is almost twice as common as the incidence at term, usually accepted as about one in 80 deliveries. When only one sac is lost early in pregnancy, with preservation of the other, this event may explain some incidences of otherwise unexplained vaginal bleeding in the first trimester in unsuspected twin pregnancies which then continue successfully with a live single fetus.