ABSTRACT
Defi nitions .................................................................................................... 109
Prevalence ................................................................................................... 109
Classifi cation .............................................................................................. 110
Aetiology ........................................................................................................ 110
Other physiology ...................................................................................... 111
Complications relevant to twin pregnancy ............................. 111
Complications unique to monochorionic twinning ............ 113
Complications unique to monoamniotic
twinning ............................................................................................... 114
Differential diagnosis ............................................................................ 114
Antenatal management ....................................................................... 114
Intrapartum management .................................................................. 116
Higher multiples ....................................................................................... 118
In 1-2 per cent of pregnancies, there is more than one fetus. The chances of miscarriage, fetal abnormalities, poor fetal
growth, preterm delivery and intrauterine or neonatal death are considerably higher in twin than in singleton pregnancies. In
about two-thirds of twins the fetuses are non-identical, or dizygotic, and in one-third they are identical, or monozygotic. In all
dizygotic pregnancies there are two functionally separate placentae (dichorionic). In two-thirds of monozygotic pregnancies
there are vascular communications within the two placental circulations (monochorionic) and in the other one-third of cases
there is dichorionic placentation. Monochorionic, compared to dichorionic, twins have a much higher risk of abnormalities
and death. The maternal risks are also increased in multiple gestations, including adverse symptoms such as nausea and
vomiting, tiredness and discomfort, and the risk of serious complications, including hypertensive and thromboembolic
disease, and antepartum and postpartum haemorrhage.
