A new analysis of 36 studies including a total of more than 12,500 women suggests these behaviors can also lower a mom-to-be's chances of having a Cesarean-section delivery or developing diabetes while pregnant.
Overall, healthy habits reduced the risk of needing a C-section by about 10 percent, said study author Shakila Thangaratinam. She's a professor of maternal and perinatal health at Queen Mary University of London.
A healthy lifestyle also reduced a woman's risk of developing diabetes during pregnancy -- known as gestational diabetes -- by 24 percent, the findings showed.
Not surprisingly, healthy habits also trimmed the possibility of excess weight gain during pregnancy.
"Based on all the evidence to date, what we found was a healthy diet and moderate physical activity in pregnancy does reduce the risk of increased [excessive] weight gain, and this benefit is actually accessible to every pregnant woman, regardless of [pre-pregnancy] body weight," Thangaratinam said.
However, it's difficult to define exactly what a healthy diet should be or what moderate exercise would be best during pregnancy, she noted, since each study looked at diet and exercise a bit differently.
In general, the diets encouraged eating more fiber, more fish and olive oil, and no sugary drinks. The studies looked at pregnant women who did stationary bicycling, swimming, dancing and toning exercises, she added.
Thangaratinam recommends 150 minutes of moderate activity a week, with aerobic exercise and two muscle-strengthening sessions.
A healthy diet and regular exercise wasn't linked to outcomes such as stillbirth or having an underweight or overweight baby, or whether an infant needed to be in the neonatal intensive care unit, the research team found.
That should be reassuring, Thangaratinam said, because many women are fearful of exercising during pregnancy, thinking it will harm the baby.
The findings are valuable, said Margie Davenport, an assistant professor of physical education and recreation at the University of Alberta in Canada, who has done research in this area. More than half of women gain above or below the weight gain recommendations, she said, and that increases the risk of C-sections, gestational diabetes and other problems.
The study reinforces both the safety and benefits of having a healthy, active lifestyle during pregnancy, Davenport said.
"Currently, only 15 percent of women are active during pregnancy due, at least in part, to a fear that continuing to exercise may be harmful," Davenport explained.
While the study results provide reassurance about the safety of exercise, "all women should speak with their health care professional prior to beginning or continuing to exercise during pregnancy," she said.
One expert noted a limitation of the study.
Eighty percent of the women included in the analysis were white and more than half were of a higher social class, said Marlene Goldman, professor emerita of obstetrics and gynecology at Dartmouth-Hitchcock Medical Center, in Hanover, N.H.
"Studies like this really don't confirm anything," Goldman said. "I would say it suggests a pretty small effect."
But she added that the findings do offer some reassurance that a healthy diet and exercise may actually confer some benefits, at least in the women studied.
In an accompanying journal editorial, Marian Knight, a University of Oxford professor of maternal and child health, and a colleague noted that maternal obesity and excessive weight gain in pregnancy are linked with complications for both mother and baby. The remaining challenge, the editorial writers said, is to research what kind of physical activity is best during pregnancy.
The study was published online July 19 in the BMJ.
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