It's estimated more than half of all twins are born early, prior to 37 weeks gestation, which is almost a month before the standard 40 weeks gestation of a singleton baby. Triplets and other higher-order multiples have an even greater chance of being born early. The reasons for preterm delivery in twin pregnancies is likely multifactorial and different from that of singletons and often due to an overdistended womb and the sheer increase in size of the womb and volume of the amniotic fluid.