Two cases of prolapse of the amniotic sac through a partially dilated cervix, the so-called “hourglass membranes,” are presented. In both cases, ultrasonography revealed the umbilical cord in the prolapsed sac. Both fetuses were grossly immature, and efforts were made to delay labor. In neither patient did rupture of the membranes occur. Fetal distress developed in 1 case, necessitating an emergency cesarean section shortly after the initiation of tocolytic therapy. A 27-week, 800-g infant was delivered who subsequently died. The second case resulted in a 3-week delay in the delivery of a viable fetus who did well. It is concluded that when confronted with a very immature fetus, aggressive measures to delay delivery are justified.