The most common complication of IVF is twin pregnancies, which occurs in 20% of pregnancies following the transfer of two embryos. Twins are associated with higher maternal and perinatal complications including miscarriage, pregnancy-induced hypertension, ante-partum haemorrhage, gestational diabetes, operative delivery, prematurity and permanent handicap in the newborn. The perinatal mortality rate in IVF twin pregnancies is six times higher than that for singletons and the risk of neurological problems, especially cerebral palsy substantially higher. Rearing of twins is also associated with practical difficulties for parents. Single embryo transfer can minimize twin pregnancies. Single embryo transfer is now being seriously considered as a means of minimizing the risk of multiple pregnancies. However, this needs to be balanced against the risk of jeopardizing the overall live birth rate.