ABSTRACT
The most common multiple is a twin pregnancy, with higher-order multiples being much rarer. The majority of twins (75%) are dizygotic (DZ) and result from the fertilisation of two ova by two sperm. Monozygotic (MZ) twins arise from the splitting of a single fertilised ovum. Chorionicity relates to the placentation of the pregnancy, and is determined by the stage at which the zygote divides. All the normal maternal physiological adaptations of pregnancy increase in multiple pregnancies, as do the risks of any pregnancy complication, such as hypertension, placental abruption and anaemia. Multiplets have a more complicated lifein utero than singletons and, as previously mentioned, monozygotic twins are at higher risk than dizygotic twins, but it is chorionicity and amnionicity rather than zygosity that determines the degree of risk. Perinatal mortality statistics do not accurately reflect the problems of multiple pregnancies, because the highest rate of fetal loss occurs prior to viability.
