The effect of vigorous exercise during pregnancy on birth-weight.

The aim of this study was to assess the effect on birth-weight of continuing a programme of vigorous exercise into late pregnancy. 'Potential exercisers' were women who had been doing vigorous exercise prior to pregnancy and intended to continue exercising during pregnancy. Controls were healthy pregnant women who did not do regular vigorous exercise. Both groups kept 2, 7-day food +/- exercise diaries at 25 and 35 weeks. The primary outcome variable was birth-weight. Women who continued doing at least 30 minutes of vigorous exercise at least 3 times weekly at 25 weeks and either maintained this minimum level or had delivered by 35 weeks were classified as 'exercisers'. Women doing more than 4 sessions of vigorous exercise weekly at 25 weeks had babies whose mean birth-weight was 315 g lower than the mean birth-weight of babies born to controls.