The average duration of a normal singleton pregnancy is 282 days. In a twin pregnancy it is 258 days. Slightly less than half of all twins are premature. A birth after about 26 to 28 weeks and up to 37 weeks is termed premature or preterm .
Prematurity is more common in twin pregnancies than in singleton.
Preterm delivery occurs in 35 percent of monozygotic twins, and in 25 percent of dizygotic twins. It has generally been accepted that the more babies there are, the earlier they could be born. Higher order multiples can not only suffer from prematurity, but low birth weights. Usually five to ten minutes are allowed to elapse before the doctor starts the delivery of the second twin. The average birth weight for singletons is 7 lbs.