OBJECTIVE: To determine whether one structurally affected fetus of a twin pregnancy further increases the risk of preterm delivery and to compare perinatal morbidity and mortality in these pregnancies with twin gestations with structurally normal fetuses. STUDY DESIGN: The cases (n = 25) included all twin gestations diagnosed from 1991 to 1994 with a sonographically detected fetal anomaly and a structurally normal co-twin delivered after 24 completed weeks' gestation. The control group consisted of 547 twin gestations delivered during the study period with no sonographically detected structural anomalies in either twin. RESULTS: Compared with controls, pregnancies with a single anomalous fetus (cases) delivered at a significantly lower gestational age (mean ± SD: 34.0 ± 3.2 weeks versus 35.6 ± 3.2 weeks; p = 0.019) and had a significantly increased preterm delivery rate (76.0% vs 55.4%; p = 0.042). There was no significant difference in the incidence of intraventricular hemorrhage or respiratory distress syndrome, yet the perinatal mortality (80.0/1000 vs 6.4/1000; p = 0.000) and the average nursery stay (45.5 ± 43.3 days versus 17.0 ± 24.0 days; p = 0.003) were significantly increased for cases compared with controls. In addition, a significantly greater birth weight discordancy (≥30%) was seen in cases compared with controls (32.0% versus 9.1%; p = 0.002). The normal co-twin did not show any significant difference in the perinatal outcome variables studied when compared with controls. CONCLUSION: Compared with structurally normal twin pairs, twin gestations with a single anomalous fetus are at a significantly increased risk for preterm delivery. In addition, the anomalous fetus, but not the structurally normal co-twin, has a significantly increased mortality rate and a longer nursery stay. Finally, despite the increased risk for preterm delivery in twin pregnancies with one anomalous fetus, it is the nature of the... [ABSTRACT FROM AUTHOR]