While babies born at 37 or 38 weeks have always been considered 'full term', scientists have now found that such tots have slightly lower IQs. They also face a modestly higher risk of death in early infancy, as compared to those born after closer to 40 weeks in the womb.
Michael Kramer, a McGill University epidemiologist, says that the finding attains significance as births these days are increasingly induced after 37 or 38 weeks of pregnancy.
He points out that it has been assumed for years that a few weeks in the final month of pregnancy do not matter much to babies, his team have now found evidence that those extra weeks can make a difference.
While making a presentation at a conference in California this week, Dr. Kramer's research associate Seungmi Yang revealed that the IQs of babies born at 37 weeks had been found to be 1.7 points lower than those of infants born at 39 or 40 weeks during their study.
Those seeing the presentation heard that the study involved 18,000 children who underwent cognitive testing at the age of six and a half.
"There was an increase in IQ from 37 to 40 weeks. The IQ score was highest at 40 weeks of gestational age," the Globe and Mail quoted Dr. Yang, who works at the Research Institute of Montreal Children's Hospital and McGill University Health Centre, as saying.
He further revealed that a similar finding surfaced when he and his colleague Xun Zhang examined the mortality rate of more 12 million babies born in the US, with those delivered at 37 and 38 weeks having a small, but significantly higher chance of dying as newborns.
The researcher revealed that infant mortality rates were highest for babies born at 37 weeks - 0.66 per 1,000 in the neonatal period and 1.68 per 1,000 in the post-neonatal period.
The rates decreased between 37 and 39 weeks, and remained stable for babies born at 40 weeks, at 0.34 per 1,000 for newborns and 1.03 per 1,000 later.
They also had an increased chance of neonatal seizures or other problems shortly after birth.
"Despite a low absolute risk of infant death at these gestational ages, the risks were more than 50 per cent higher at 37 weeks than at 40 weeks," the researchers say in a research article, published in the Journal of Pediatrics.
Dr. Kramer conceded that those differences were relatively small, but insisted that they raised serious questions about whether inducing births at 37 and 38 weeks does more harm than good.
He stressed the need for a large clinical trial that would compare the outcomes of term births induced at different gestational ages for a variety of reasons and births that weren't induced in similar circumstances.