Multiple pregnancy is when there is more than one foetus. Two babies are twins, three babies are triplets, four babies are quadruplets and five babies are quintuplets and so on.

Twins can be mono-zygotic (MZ) (from one zygote – where an embryo splits) or di-zygotic (DZ) where they arise from two different fertilised oocytes. They can be mono-chorionic or di-chorionic. Chorionicity is critical for management. Mono-chorionic twins are always mono-zygotic.

Chorionicity is determined by ultrasound in the first trimester. Mono-chorionic twins need much closer surveillance. Most mono-chorionic twins are di-amniotic, each with its own amniotic sac, but occasionally they are mono-amniotic.

The incidence of natural twins is 1:90, triplets is 1:90 × 90 = 1:8,100, and quads 1:90 × 90 × 90 = 1:729,000.

During the last few decades the incidence of multiple pregnancies has risen as a result of reproductive technology, ovulation induction and in vitro fertilization.

Di-zygotic twins occur as a result of superovulation, but can also be due to the transfer of multiple embryos, resulting in non-identical twins. Monozygotic twins are due to the embryo splitting. It is suggested that embryos transferred during IVF at the blastocyst stage have a higher incidence of monozygotic twinning (incidence 2 %).

Whether twins are mono-amniotic or bi-amniotic, mono-chorionic or bi-chorionic depends on the gestation at which the embryo splits.