The "vanishing twin": ultrasonographic assessment of fetal disappearance in the first trimester.

Review of the sonographic findings of 1000 pregnancies with viable gestations in the first trimester revealed a minimum incidence of twinning of 3.29%. Of these, 21.2% demonstrated the "vanishing twin" phenomenon, often with associated bleeding, but with a good prognosis for the remaining fetus. Ovulation induction did not appear to alter these data. The sonographic criteria for "vanishing twin" are discussed. The incidence of multiple gestation is 3.29% to 5.39%, higher than previously believed.