Babies born at 37 or 38 weeks are considered to be full term, but new research has found that they have slightly lower IQs and a modestly higher chance of dying in early infancy than those who arrive after closer to 40 weeks in the womb.
The findings are troubling because an increasing number of births are induced after 37 or 38 weeks of pregnancy, said Michael Kramer, a McGill University epidemiologist, who is scientific director of Canada’s Institute of Human Development, Child and Youth Health, part of the Canadian Institutes of Health Research.
These babies are not premature. Any infant born between 37 and 41 full weeks of pregnancy is considered a term baby. For years, doctors and researchers have assumed that a few weeks in the final month of pregnancy didn’t matter much to babies, although they can be tough on the moms-to-be.
“Most doctors and mothers think that, once you reach 37 weeks, all is fine,” said Dr. Kramer, a professor of pediatrics and of epidemiology and biostatistics at McGill. But he and his colleagues have found evidence that those extra weeks can make a difference.
At a conference in California this week, his research associate, Seungmi Yang, presented the results of study that found that babies born at 37 weeks had IQs that were 1.7 points lower than those of infants born at 39 or 40 weeks. 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,” said Dr. Yang, who works at the Research Institute of Montreal Children’s Hospital and McGill University Health Centre.
Dr. Kramer and his colleague Xun Zhang made a similar finding when they examined the mortality rate of more 12 million babies born in the United States. Those delivered at 37 and 38 weeks had a small, but significantly higher chance, of dying as newborns.
Most doctors and mothers think that, once you reach 37 weeks, all is fine — Dr. Michael Kramer
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,” they concluded in the article, which was published in the Journal of Pediatrics.
These differences are relatively small, said Dr. Kramer, but raise serious questions about whether inducing births at 37 and 38 weeks does more harm than good.
The rate of induced births doubled in the 1990s, but the practice is not done for the convenience of the doctor or the mother, said Dr. Kramer.
“Most of the time, doctors are worried that something is not quite right,” he said. Perhaps the baby isn’t growing quite as well, or the mother feels less kicking. Her blood pressure or blood sugar might be too high.
Doctors are usually trying to prevent a stillbirth, or complications for the mother. It is well-meant, said Dr. Kramer, but may not be what is best for the baby. What’s needed is a large clinical trial that compares the outcomes of term births induced at different gestational ages for a variety of reasons and births that weren’t induced in similar circumstances, he said.
