Multiple pregnancy refers to the situation where there is more than one foetus present in the uterus. The incidence of this is roughly 1 in 100 for twin pregnancies, and 1 in 10,000 for triplets - though the incidence is increasing as a result of assisted conception. Twins are said to be either monozygotic (where the two babies develop from the same fertilised egg and are therefore identical) or dizygotic (were two separate eggs have been released and fertilised by two separate sperms, so the two babies are not necessarily any more like each other than any other children of the family). Twin pregnancies are more likely in people who have previously had twins or where there is a family history of twins. It’s also slightly more likely in the older mother. Before the days of ultrasound this often went undiagnosed (the unexpected arrival causing surprise at delivery) but now this is highly unlikely. Multiple pregnancies have a higher incidence of pregnancy complications such as increased nausea and vomiting, pre-eclampsia and premature birth, and need careful monitoring.