Here's important news for couples planning a family: Women who space their pregnancies close together or far apart are at an increased risk of giving birth prematurely, and having a baby with a low birth weight who's smaller.
Researchers reviewed data spanning more than 30 years and found that if the time between pregnancies was less than 18 months or more than five years, the babies born during the second pregnancy were at a higher risk of having difficulties early in life. The analysis was published in this week's issue of the Journal of the American Medical Association.
The researchers said the reasons for the risk are unclear. They could be related to nutritional depletions in the mother, or even something more difficult to describe, such as lifestyle, socioeconomic status or psychological factors.
But it is an especially important finding for women who have waited until their late 30s to start a family. As women get older, their fertility declines, making some women in this group rush to have all their children quickly.
"The reality is that with women working, many may feel that they simply may not have time to be out of the work force for years and may consolidate the time that they have small children," said Dr. Nathana Lurvey, an obstetrician at Cedars-Sinai Medical Center. "This is not in the best interests of maternal or child health. But in a society that does not support women as mothers or children as infants -- this is the reality."
However, with data clearly showing this can be harmful, health care providers say women need to plan ahead.
"I advise women of all ages to consider their reproductive life plan -- in which they think about whether or if they want to have a child or children, when the ideal time would be to have children, how many children they would ideally like to have," said Dr. Paula J. Allan Hillard at the University of Cincinnati.
At the very least, women who have had smaller babies or growth problems during pregnancies in the past should not get pregnant for at least six months after a child is born, said Dr. Jacques Moritz, an obstetrician at Columbia University in New York.
One way to do this is to breast-feed, since it's not only good for the baby, it temporarily reduces a woman's fertility, Lurvey said.
And while spacing out pregnancies is important, there are other factors that affect a baby's health, doctors noted. For example, it's helpful to take prenatal vitamins during pregnancy when attempting to get pregnant and when breast-feeding.
Doctors also need to follow pregnancies closely to look for signs of growth delays in the fetus.
"Older women who are health-conscious take prenatal vitamins with folate when attempting pregnancy, plan their pregnancies, and are well-educated and informed will almost certainly have better pregnancy outcomes than young women with unplanned pregnancies, even with shorter interpregnancy intervals," Hillard said.
Dr. Khama Ennis-Holcomb is an emergency medicine resident at Massachusetts General and Brigham & Women's Hospitals in Boston.