Twin pregnancy

Twin pregnancies occur in ~12 per 1,000 pregnancies worldwide and are associated with substantially increased maternal, fetal and neonatal morbidity and mortality compared with singleton pregnancies. Although zygosity describes whether twins arise from one fertilized egg (monozygotic) or two fertilized ova (dizygotic), clinical risk is primarily determined by chorionicity (whether twins share a placenta) and amnionicity (whether twins share an amniotic sac). Accurate early determination of chorionicity and amnionicity is therefore fundamental to counselling, risk stratification, surveillance and management, and should be established and documented consistently throughout pregnancy. Monochorionic twins are uniquely susceptible to complications arising from shared placental vascular anastomoses, including twin-twin transfusion syndrome and twin anaemia-polycythaemia sequence or unequal placental sharing in the form of selective fetal growth restriction. Preterm birth remains the leading cause of perinatal morbidity and mortality in twin pregnancies, with the risk of extreme preterm birth markedly higher than in singleton gestations. Contemporary surveillance strategies aim to identify evolving complications sufficiently early to enable timely intervention and optimize outcomes.