Later splitting will result in conjoined twins. Monozygotic pregnancies have random geographic distribution . Dizygotic pregnancies occur when two eggs are fertilized. Polyovulation may be genetically determined, occur after discontinuation of contraception, or induction of ovulation. Multiple gestation is also more common after invitro fertilization and embryo transfer, hence unequal geographic distribution.
Risks associated with prematurity, which is more frequent in multiple gestation, are most important.
Also, risks associated with common abnormal fetal lies and presentations. Several other complications of pregnancy are more common in twin gestation. In addition, monochorionic placentation is associated with acute and chronic twin transfusion syndrome. Monoamniotic pregnancies have increased frequency of cord entanglements between the twins. They occur only in monochorionic placentas with vascular anastomoses.