and a composite score of perinatal morbidities were compared between uncomplicated twins that had a planned preterm delivery and monochorionic twins that continued in utero >34 weeks of gestation, and dichorionic twins who continued beyond 36 weeks. Twenty-seven recruited patients did not complete the study. A total of 200 of the remaining 1001 patients were designated as monochorionic and 801 of 1001 as dichorionic twin pairs. Mothers of dichorionic twins were older (31.3 vs. 33.0; P<0.001) and were more likely to have used assisted conception techniques than mothers of monochorionic twins (28% vs. 3%; P<0.001). Common antenatal obstetric complications were equally prevalent in monochorionic and dichorionic pregnancies. The perinatal mortality rate was 30/1000 for monochorionic neonates [95% confidence interval (CI) upper limit 53/1000] and 3.8/ 1000 for dichorionic neonates (CI upper limit 10/1000). Previable dual or single in utero fetal death was more common among monochorionic gestations. Overall, 80% and 93% of monochorionic and dichorionic pregnancies, respectively, delivered after 32 weeks of gestation. Between 24 and 28 weeks gestation, 9 of 200 (5%) and 12 of 801 (1.5%) monochorionic and dichorionic twins, respectively, were delivered. At 34 weeks, 131 of 200 (66%) of monochorionic twins were considered to be in an uncomplicated pregnancy and at 36 weeks, 565 of 801 (71%) of dichorionic twins were considered uncomplicated. For uncomplicated, monochorionic pregnancies, the prospective risk of in utero death was 1.5% after 34 weeks gestation; no deaths occurred among dichorionic twins after 33 weeks. The morbidity in uncomplicated monochorionic twin pairs who continued in utero and were delivered either electively or for maternal/fetal indications was 5% (2/44), 9% (7/82), and 5% (4/84) for neonates delivered at 35, 36, and 37 weeks gestation, respectively. The risk of composite perinatal morbidity for uncomplicated monochorionic twins decreased from 41% (13/32 neonates) at 34 weeks to 5% (4/ 84) at 37 weeks [odds ratio (OR), 13.5; 95% CI, 2.5-72.4; P<0.001]. The neonatal intensive care unit admission rate fell from 88% at 34 weeks to 9% at 38 weeks (28/32 and 2/ 22, respectively; P<0.001). For dichorionic twins, no deaths occurred after 33 weeks gestation. For those delivered electively at 36 and 37 weeks, no difference in morbidity was seen relative to neonates delivered after 37 weeks (OR, 1.61; 95% CI, 0.4-7.1; P=0.52) and among electively delivered uncomplicated dichorionic twins delivered at 37-38 weeks relative to after 38 weeks (OR, 2.5; 95% CI, 0.8-7.5; P=0.09). The overall mortality and morbidity among neonates in this study were low. For both types of twins, most fetal deaths occurred either before viability or at a gestational age so remote from term that elective preterm delivery would not have been an appropriate action. With close fetal surveillance, perinatal morbidity can be minimized by recognizing the balance present between fetal risk and neonatal risk when considering elective preterm delivery. Prenatal, Perinatal, and Neonatal Risk Factors for Autism