New IOC Paper Weighs in on Training
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Mothers who exercise during pregnancy are less likely to have large babies, i.e., greater than 8.8 lbs, and they are not at increased risk for having small babies (under 5.5 lbs).
Clinical trials examining the effect of exercise on newborn birth weight have prescribed exercise regimens of at least two or three 30-minute sessions per week, with the result being usually a minor reduction in birth weight. There are no published trials evaluating newborn birth weight in elite athletes, and the researchers caution extrapolating outcomes for elite-level training from the far more modest regimens prescribed in the clinical trials; elite athletes obviously exercise at a much higher frequency and intensity. In any case, reducing birth weight to below 8.8 lbs reduces the risk of obstructed labor, caesarean section and childhood obesity. Additionally, the IOC statement reports that physical activity during pregnancy either decreases the duration of labor or has no effect on it.
Preterm birth is defined as birth of a live-born infant prior to the completion of the 37-week gestation period. Exercise during pregnancy does not appear to increase the risk for premature birth or induced labor.
In 2010, a Cochrane review of 14 randomized clinical trials compared women who performed aerobic exercise with those who did not, but the pooled estimate from the relatively small trials was not statistically significant to establish results on preterm birth. Since then, at least six randomized trials have been published, with sample sizes ranging from 3,554 to 32,055 pregnant women.
In five of these more recent studies, 55 to 60 structured aerobic exercise classes were offered from early to late pregnancy. Women in the intervention groups were compared to a usual care control group, with no differences in preterm birth rates between the groups in any study. In another study, sedentary women assigned to water aerobics had a similar risk of preterm birth to sedentary controls.
There is moderately good evidence, then, that there are few effects of exercise on preterm birth rate in the general population. There have been no studies of elite athletes.