The effects of vigorous exercise during pregnancy on birth weight.

Whether or not vigorous exercise during pregnancy affects birth weight is an important issue because birth weight is the single most important determinant of neonatal mortality. A current meta-analysis of randomised trials of exercise during pregnancy did not find, although also did not exclude, a clinically important effect of exercise on birth weight. Randomised trials may miss the women exercising at the most elite levels during pregnancy. We should remain open-minded on this issue until sufficient evidence is available on which to base recommendations.