Generally, twins and other multiples are born earlier than singletons. The risk of preterm labor and premature birth are heightened when there is more than one baby in the womb. Other conditions such as preeclampsia, placental dysfunction, and TTTS are more prevalent in twin pregnancy and increase the risk of an early delivery.
Research from the National Organization of Mothers of Twins Clubs (NOMOTC) indicates that about half of multiples are born before 36 weeks gestation. An informal, unscientific poll on my site indicates 18% delivered before 28 weeks, and only 15% after 38 weeks. The National Vital Statistics Report for 2009 reports that 60% of twins were born preterm (less than 37 completed weeks of gestation) and 11.4% were considered very preterm (less than 32 completed weeks of gestation).
There's no way to know exactly when your babies will be born. In some cases, a premature birth can not be avoided. Some doctors feel that thirty-seven or thirty-eight weeks should be considered full-term for twins and will seek to prompt the delivery of the babies at that time. A 2006 study found that twins delivered past 40 weeks were more likely to have lower APGAR scores and a higher mortality rate, however, it also concluded that there were no additional health benefits for babies delivered at thirty-seven or thirty-eight weeks. However, in 2012 researchers from Adelaide University concluded that thirty-seven weeks was the ideal gestation for twins, citing that babies born after that point faced increased risk of slowed growth, resulting in low birth weight and other potential health complications. You should consult with your medical caregivers to determine the best course of action for your pregnancy and delivery, to ensure the healthiest outcome for both mother and babies.
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