For otherwise healthy twin pregnancies where both twins are head down a trial of vaginal delivery is recommended at between 37 and 38 weeks. Vaginal delivery, in this case, does not worsen the outcome for either infant as compared with caesarean section. There is some controversy on the best method of delivery where the first twin is head first and the second is not, but most obstetricians will recommend normal delivery unless there are other reasons to avoid vaginal birth. When the first twin is not head down, a caesarean section is often recommended. Regardless of whether the twins are delivered by section or vaginally, the medical literature recommends delivery of dichorionic twins at 38 weeks, and monochorionic twins (identical twins sharing a placenta) by 37 weeks due to the increased risk of stillbirth in monochorionic twins who remain in utero after 37 weeks. The consensus is that late preterm delivery of monochorionic twins is justified because the risk of stillbirth for post-37 week delivery is significantly higher than the risks posed by delivering monochorionic twins near term (i. e. , 36–37 weeks). The consensus concerning monoamniotic twins (identical twins sharing an amniotic sac), the highest risk type of twins, is that they should be delivered by caesarean section at or shortly after 32 weeks, since the risks of intrauterine death of one or both twins is higher after this gestation than the risk of complications of prematurity.