Monoamniotic twins are uncommon but are at high risk (reportedly 50%) for perinatal death, commonly from cord accidents. Until recently the diagnosis of monoamniotic twinning was seldom made before delivery, but modern ultrasound technology permits diagnosis during prenatal care, creating a management dilemma. This is a report of the experience with monoamniotic twins of 20 or more weeks' gestation at the University of Iowa Hospitals from 1961- 1989. Twenty monoamniotic twin pregnancies were compared with 40 monochorionic, diamniotic controls regarding antepartum and intrapartum complications. Overall, monoamniotic twins were delivered earlier, were more likely to die in utero, and had lower birth weights than diamniotic twins. When only live-born twins were considered, however, there were no differences in gestational age at delivery, birth weight, or 5-minute Apgar scores. No fetal death occurred after 32 weeks, suggesting that prophylactic preterm delivery may not be indicated in all cases. Labor and vaginal delivery were not associated with an increased risk of fetal death. (Obstet Gynecol 77:832, 1991)