Even when babies are born "full term," those who spend fewer weeks in the womb may still be less likely to earn a college degree and get a high-paying job, a large study suggests.
In the analysis of 228,030 singleton births in Denmark, infants who were born after only 38 weeks of pregnancy were 15 per cent less likely than those born at 40 weeks to have some education beyond high school or to be among the top earners in the study.
"While adults born at 35 to 38 weeks of gestation experienced only slightly lower chances of high income and a high educational level, this may have a significant impact since a large proportion (approximately 10 per cent) of all children are born in these weeks," said lead study author Josephine Funck Bilsteen of the University of Copenhagen and Hvidovre University Hospital in Denmark.
Pregnancy normally lasts about 40 weeks, and babies born after 37 weeks of gestation are considered full-term. Babies born prematurely - earlier than 37 weeks - often have difficulty breathing and digesting food in the weeks immediately following birth. These preemies can also encounter longer-term challenges such as impaired vision, hearing and cognitive skills, as well as social and behavioural problems.
"Previous research has shown that adults born preterm are more likely to have a lower educational level and lower income than adults born term," Bilsteen said by email. "However, much less is known about differences in education and income among adults born at different gestational weeks within the term period."
To examine this question, researchers looked at data for babies born in Denmark between 1982 and 1986.
As expected, the earliest preemies fared the worst. Compared with infants born at 40 weeks, babies who arrived at just 22 to 27 weeks gestation were 79 per cent less likely to have any education beyond high school and 34 per cent less likely to be among the top wage earners in the study, researchers report in JAMA Network Open.
Just one week might make a difference. Babies born at 39 weeks were still slightly less likely to succeed academically or financially than infants who arrived at 40 weeks.
These estimates accounted for other factors that can independently impact both gestational age and adult achievement including sex and birth year as well as mothers' age, education levels and country of origin.
The study wasn't designed to prove whether or how gestational age might directly impact educational or economic outcomes in adulthood, however. It's possible, for example, that gestational age reflects some other individual or family factor that also influences socioeconomic outcomes.
One limitation of the study is the potential for gestational age to be inaccurate for some babies since doctors can use different methods to estimate when a baby was conceived and might misclassify infants in some cases, the study authors note.
"I'd suggest that parents take these findings with a grain of salt," said Margaret Kern, a researcher at the University of Melbourne in Australia who wasn't involved in the study.
"Early birth does not doom a child to failure in the world if that is defined as economic success, which I'd question," Kern said by email. One in five babies arrive early and many of them do well in adulthood, she added, "so clearly there are a lot of success cases there to draw on."
While parents can't control when babies arrive, they should keep the potential for negative outcomes in mind when scheduling an elective cesarean section delivery, said Dieter Wolke, a researcher at the University of Warwick in the UK who wasn't involved in the study.
Babies that come early, even on the early end of what's considered full-term, may be more shy or withdrawn and struggle with attention and participation in school, Wolke said by email.