A full-term pregnancy usually lasts about 39 to 40 weeks, and it’s hardly a secret that a preterm-birth can result in some serious health complications for the baby. What’s less known is that keeping a baby in the oven for too long can also increase the risk of health issues, including stillbirth. The long-term cognitive and physical outcomes associated with late-term gestation, however, have long been unknown, at least until now.

These effects were the focus of new research published in JAMA Pediatrics. To figure out the cognitive and physical effects of late-term birth, researchers from Northwestern University compared the birth certificates of 1.4 million children in Florida to their school records. Each child was born at 37 to 41 weeks of gestation. They compared the data on late-term babies (born at 41 weeks) to those born at full term (39 or 40 weeks) with three school-based cognitive measures and two physical outcomes — abnormal newborn conditions and physical disabilities noted in the school record.

Their results showed that late-term infants confer some benefit when it comes to cognitive function. The data showed that those born at 41 weeks had higher average test scores in elementary and middle school, and a 2.8 percent higher probability of being gifted, as well as a 3.1 percent reduced probability of poor cognitive outcomes compared to those born at full term.

However, late-term infants didn’t fare as well as full-term infants when it came to physical functioning. The study found that late-term infants had higher rates of abnormal conditions at birth, such as respiratory disease/distress, as well as a 2 percent higher chance of physical disability by the time they reached 5 or 6 years old.

"In summary, these findings suggest there may be a tradeoff between physical and cognitive outcomes associated with late-term gestation,” the researchers concluded. “While late-term gestation was associated with an increase in the rate of abnormal conditions at birth and with worse physical outcomes during childhood, it was also associated with better performance on all three measures of school-based cognitive functioning measures during childhood."

It’s possible that late-term infants had higher levels of cognitive function because their brains had more time to develop while in the womb. According to March of Dimes, some of the most important things that happen to a baby in the last few weeks of a full-term pregnancy include the continued development of the baby’s brain and lungs. These organs may continue to develop into late term.

While the findings don’t provide a course of action for physicians, they do suggest that there may be some benefits to continuing pregnancy beyond 40 weeks, the researchers said, adding that it's useful for “expectant parents and physicians who are considering whether to induce delivery at full term or wait another week until late term.”

Source: Figlio D, Guryan J, Karbownik K, Roth J. Long-term Cognitive and Health Outcomes of School-Aged Children Who Were Born Late-Term vs Full-Term. JAMA Pediatrics. 2016.