Medical complications are more common in a multiple pregnancy than in a pregnancy with one fetus. They may include:

• Vanishing twin syndrome early in the pregnancy. About two-thirds of twin pregnancies are naturally reduced to one fetus in the first trimester.¹ The mother and remaining twin are not harmed.
• Differences in size between the fetuses (discordance), usually from twin-to-twin transfusion.
• Higher risk of birth defects and genetic disorders.

Rare problems include:

• Twins that share one amniotic sac (monoamniotic twins).
• The fetuses becoming entangled (locking twins) during delivery.
• Conjoined twins. Twins who are joined together (such as at the chest, head, or pelvis) are called conjoined (Siamese) twins. Conjoined twins are very rare and result when the cells of identical twins do not divide correctly.