In general the more babies in a multiple gestation the earlier the delivery is going to be, and that's usually due to spontaneous preterm birth which occurs without any inciting factor. The average gestational age for singleton gestation in United States is little over 39 weeks. But for twins it's about 35 weeks; triplets, 32 weeks; quadruplets, 29 weeks and even earlier for higher order multiples. Although that's for spontaneous preterm birth, multiple gestations are also at risk for indicated preterm birth due to problems with the baby's growth or moms might develop hypertension which is more common and they might have to be delivered even earlier because of those type of complications.
