average twin birth weight
1 The average duration of a regular pregnancy is 40 weeks. In a twin pregnancy. the average is 37 weeks. for triplets 34 and quadruplets 32 weeks.  The average birth weight for singletons is 7 lbs 7 oz. Average birth weight of twins is 5 lbs 5 oz. for triplets 4lbs and quads 3lbs.