The mean gestational age for a twin pregnancy is 37 weeks, for triplets 34 weeks and for quadruplets 31 weeks. Compared to singletons the average birth weight for multiples is much lower: at birth the mean weight for twins is 2500 g, for triplets 1800 g and for quadruplets 1400 g. The lower the gestational age, the higher the chance of Neonatal Intensive Care (NIC) due to extreme low birth weight (<1500g). This counts for 8% of all twins, 30% of all triplets and 55% of all quadruplets.
|mean gestational age||37 weeks||34 weeks||31 weeks|
|average birth weight||2500 g||1800 g||1400 g|
|chance of NIC||8%||30%||55%|
During the first 28 weeks of the pregnancy twins or multiples grow at the same rate as singletons do. Past 28 weeks de growth of the fetuses usually decreases. This can be caused by less space in the womb or the size of the placentas. So the third trimester of the pregnancy can be a risky period. It is recommended to take enough rest during this period to limit the risk of early birth.
As a matter of fact there is also a maximum gestation for twin or multiple pregnancies. Because at the end of the pregnancy the chance of complications increases. For a twin pregnancy this is after 38 weeks and for triplets after 37 weeks. For the health and wellbeing of the mother it usually is undesirable if the pregnancy lasts any longer. Therefor you should consider inducing delivery at 38 weeks for twins and at 37 weeks for triplets.
More about twin pregnancies in this video: http://www.twinsvideoblog.com/2011/06/07/twin-pregnancy-timeline-what-to-expect/