Women having their first child can lower the chances of having an overweight baby with regular exercise during pregnancy, researchers say.
Working out at least three times per week reduced the odds of delivering a newborn with excessive birth weight (more than 8 pounds, 13 ounces), by about a quarter, Katrine Mari Owe, and colleagues reported in the October issue of Obstetrics & Gynecology.
The birth weight- exercise associations weren't as strong for women who had given birth before -- they were more likely than first-time mothers to have an overweight baby.
Research has shown that regular exercise is an important part of a healthy pregnancy, but results of studies involving physical activity and mean birth weight have been inconsistent.
To clarify the issue, the researchers analyzed data from the Norwegian Mother and Child Cohort Study of 36,869 normal, single pregnancies. They collected data on newborn birth weight from the Medical Birth Registry of Norway.
In their study, the average birth weight was just shy of 8 pounds, 2 ounces, and 11 percent of newborns were classified as being in the 90th percentile or more of birth weight, categorizing them as "excessive."
For first time moms, there was an association between regular exercise -- at least three times per week -- and a reduced risk of an overweight baby.
Risk for "excessive" weight newborns fell by 28 percent for those who were exercising at week 17 of their pregnancy and by 23 percent for those who worked out at week 30.
The risk of having an overweight baby also trended downward with exercise among women in their second or later pregnancies, but the difference did not reach statistical significance, the researchers said.
Among those experienced moms, the risk of an overweight baby was lowest among those who danced for exercise at week 17 of their pregnancies and those who engaged in low-impact aerobics.
On the other hand, women in second-or-later pregnancies were more likely to have heavy babies if they trained in fitness centers during week 17 or swam during week 30.
For first-time mothers, women, walking decreased risk at weeks 17 and 30, as did running at week 17.
One possible explanation for exercise lowering risk of excessive birth weight among newborns is the effect of aerobic exercise on glucose tolerance, the researchers said.
Research has shown that moderate physical activity during pregnancy can lower glucose levels in both disease-free women and those who have gestational diabetes.
The study also found that exercising for three months prior to pregnancy didn't have any effect on the odds of having heavier babies for either nulliparous (first-time mothers) or multiparous women (second-time or more mothers), the researchers said.
Still, obstetricians who weren't involved in the study said the conclusion about pre-pregnancy exercise shouldn't affect a woman's pre-conception planning.
"Women considering conceiving should try to be in the best health possible and should incorporate regular exercise as part of their preparation for pregnancy," said Dr. Jennifer Wu of Lenox Hill Hospital in New York.
"As obesity and childhood obesity become increasingly problematic, the issue of excessive newborn birth weight seems to be a key issue," Wu said. "Excessive newborn birth weight … can also have severe consequences on maternal and newborn health."
The researchers noted that their study was limited by self-reported exercise data and by a low response rate in the Norwegian Mother and Child Cohort Study.
The women who elected to participate had a slightly different age distribution, were less likely to smoke and less likely to have a preterm birth.