Multiple pregnancy

Aetiology and incidence

Dizygotic twinning arises when two separate ova are fertilised and implanted. This results in non-identical twins with separate amniotic sacs, chorions and placentae (although the placental masses may fuse together). Monozygotic twinning arises when a single embryo splits to yield genetically identical twins. Depending on the timing of division relative to conception, it may result in diamniotic, dichorionic pregnancy (~33%), a diamniotic, monochorionic pregnancy (~66%), a monoamniotic, monochorionic pregnancy (~1%) or conjoined twins (rare). Chorionicity may be accurately determined by ultrasound at 11–14 weeks' gestation, but becomes more difficult thereafter. The incidence of monozygotic twinning is relatively stable worldwide at around 3.5 per 1000 births, whilst the incidence of dizygotic twining varies with race and maternal age (~8.5 per 1000 births in Europe and North America) (Little and Thompson 1988). Dizygotic twinning may also result from assisted conception techniques, leading to a higher incidence in some centres. For the UK as a whole, the incidence of multiple births has increased from 10.4 per 1000 maternities in 1985 to 14.4 per 1000 in 1997, with higher-order pregnancies tripling from 0.14 to 0.45 per 1000, reflecting the increasing use of assisted conception techniques (Taylor and Fisk 2000).

Preterm delivery is considered the most important complication of multiple pregnancy and it is responsible for the majority of the increased perinatal morbidity and mortality. The Scottish Twins Study (Patel et al. 1984) reported delivery before 37 completed weeks in 44% of twins (vs. 6% of singletons), and delivery before 32 completed weeks in 10%.