Objective: To assess parameters associated with perinatal and neonatal morbidity and mortality in monochorionic (MC) diamniotic twin pregnancies after 25 weeks of gestation. Methods: Neonatal outcomes after 25 weeks' gestational age were retrieved for all twin pregnancies between the years 2001 and 2007 and documented according to chorionicity. The features of MC pregnancies that resulted in intrauterine or neonatal demise were reviewed carefully to find an explanation for the adverse outcome. Results: The features of 93 pairs of MC twins were recorded and compared with those of 428 consecutive pairs of dichorionic (DC) twins. Perinatal mortality was higher in the MC twin pregnancies (1.61% vs. 0.35%, P= 0.04). Average gestational age at delivery (34.8 vs. 35.9 weeks, P< 0.001), mean birth weight (2127 g vs. 2443 g, P< 0.001), and birth-weight adjustment for gestational age were all lower in the MC twin group than in the DC group. Severe discordant growth of the twins (>. 20% difference in birth weights) was significantly more common in MC pregnancies (25.81% vs. 11.68%, P< 0.001). After excluding cases of discordant growth, gestational age at delivery was not significantly different between the two groups. MC twins were more likely to have lower Apgar scores (8.5 vs. 8.9 at 5 minutes, P= 0.007), a higher rate of congenital malformations (13.98% vs. 8.18%, P= 0.01), higher rates of admission to the NICU (55.91% vs. 36.57%, P< 0.001), and higher rates of velamentous cord insertion (8.60% vs. 4.56%, P< 0.001). Conclusion: Even in the presence of apparently normal progress, MC twin pregnancies should be monitored frequently throughout pregnancy, including during the third trimester.