Twin pregnancy has a disproportionate effect on perinatal mortality, being six times higher than for singleton gestations. The major threats to perinatal survival are from two very different pathological processes: spontaneous preterm delivery, and the interlacing clinical complications of monochorionicity. With the realization that perinatal loss/handicap is higher in monochorionic than dichorionic twins, attempts have been made in the last decade to assign chorionicity ultrasonographically, using single or composite parameters, such as number of placental masses, fetal sex, septal thickness and twin peak signs. Such knowledge will allow (a) risk stratification of twin gestations, (b) appropriate selection of prenatal screening and diagnostic methods, (c) vigilant monitoring for early diagnosis of twin-twin transfusion syndrome and growth restriction, and (d) management of preterm labour, congenital malformation, single intra-uterine death and polyhydramnios. By contrast, prospective knowlege of zygosity is unlikely to influence perinatal outcome, since approximately 25 per cent of monozygous conceptions have dichorionic placentation. Postnatal determination of zygosity in like sex twin pairs with dichorionic placenta is important for the future consideration of organ transplantation compatibility and to evaluate the genetic component of various diseases.