The Effect of Vigorous Exercise During Pregnancy on Birth‐Weight

Abstract

EDITORIAL COMMENT: When this paper was reviewed the question was raised that the period of gestation is an obvious factor which can determine birth‐weight and the authors were asked how this may have affected their results The authors response was as follows: The mean gestation at delivery in the 2 groups was not different statistically ‐ controls n = 41, mean = 39.96 weeks, standard deviation = 1.37 weeks; exercisers n = 58, mean = 39.48 weeks, standard deviation = 1.94 weeks. We were interested in the finding in our study that preterm delivery occurred in 4 of 58 exercisers and only 1 of 41 controls. However, the number of preterm deliveries in this study is small (a total of 5) and as the calculation of sample size was based on a shift in mean birth‐weight, this is the outcome which should be emphasized. Clearly, gestation is a very important determinant of birth‐weight. However, in terms of trying to determine whether exercisers deliver earlier, it would not help to put gestation in our analysis of variance and covariance. Gestational age would need to be the dependent variable for such an exercise and, for meaningful results, would require a larger study. Summary: 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.