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 than12 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.
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 deliveries are not usually induced 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.
That is also the only way to determine whether the potential problems that lead obstetricians to induce births - for example, a mother's high blood pressure - are related to the increased mortality and medical problems documented in his recent study.
Preterm births - babies born before 37 weeks - account for most infant deaths and for most stillbirths, Dr. Kramer said. Even babies born a few weeks early, at 34 to 36 weeks, have slightly higher risk of dying than term babies. Studies have also found they have a modest, but significant, increased risk of developmental delays and mental retardation.
Babies born late, at 42 or more weeks, also have a higher risk of dying or having health problems as newborns.
But very few researchers have looked at differences within the category of term births - babies born between 37 and 41 weeks.
One study found the risk of sudden infant death syndrome deceased with each advancing week for term babies.
In 2007, a team of Swedish scientists reported that young adults who had been born at 37 to 38 weeks had a higher risk of disability and were more likely to have a lower disposable income than people who spent more time in their mother's womb.
TIME IN THE WOMB MAKES A DIFFERENCE
How is gestational age
calculated?
From the first day of the mother's last menstrual period, in complete weeks. For example, 40 completed weeks of gestational age consists of 280 to 286 days. If that date isn't known, it can be based on ultrasounds or estimated by the doctor at birth.
Findings on infant
mortality
The babies: 12.7 million white, non-Hispanic babies born in the United States at gestational ages from 37 to 41 weeks. The results were replicated in a follow-up study on non-Hispanic blacks.
The results: 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 for babies born from 37 to 39 weeks, and remained stable at 39 to 40 weeks.
Other factors taken into account: The mother's age, marital status and education and medical risk factors, including diabetes, smoking and high blood pressure. A third sample, a subgroup of 5.7 million babies born to women without any of these health problems, found similar results.
Findings on intelligence
The babies: 18,000 children in Belarus that were born at term and beyond, all healthy. They underwent cognitive testing as six-year-olds.
The results: Those who were born at 37 weeks had IQs that were on average 1.7 points lower. There was an increase in IQ for every week from 37 to 40 weeks.
Other factors taken into account: Socioeconomic status, the mother's age at the birth, maternal smoking and drinking during pregnancy, maternal and paternal occupation, and number of children and marital status.
Anne McIlroy