In this large prospective study of Norwegian women, we found no clear association between reported leisure time physical exercise level before pregnancy and offspring birth weight. There was some evidence that inactive women had a slightly lower likelihood of giving birth to a child with excessive birth weight than more physically active women, but the small numbers of inactive women call for a cautious interpretation. Analysis of the combined association of BMI and exercise on birth weight showed that women with a BMI ≥ 25.0
gave birth to infants with significantly higher birth weight than women with a BMI < 25.0
, but only if they also reported no or low levels of physical exercise. This could suggest that physical exercise to some extent reduces the effect of maternal adiposity on offspring birth weight.
The suggestive evidence that women who were inactive before pregnancy had lower risk for delivering a macrosomic infant is contradictory to some previous studies. Voldner et al. [20
] reported that inactive women (defined as <1
h per week) had almost a threefold higher odds ratio for fetal macrosomia than physically active women (>1
h per week). However, another Norwegian study found no association between frequency of regular exercise before pregnancy and offspring with excessive birth weight (≥90th percentile) [18
]. In the present study, those who were classified as inactive reported never engaging in physical exercise, and these women could be more extremely sedentary than inactive women in other studies. Studies have shown that a sedentary lifestyle in pregnancy is associated with lower birth weight [32
] and an increased risk of a very low birth weight infant [16
]. It has been observed that mothers of very low birth weight infants were less likely to be physically active during their pregnancy [33
]. It has been suggested that both excessive and insufficient physical activities in pregnancy are related to an inadequate fetal growth [34
], although some women might be advised to be inactive and at rest to reduce the risk of adverse pregnancy outcomes.
Unlike the present study, some previous studies have shown inverse associations between maternal pre-pregnancy exercise behaviors and offspring birth weight or risk of having excess weight [17
], although the results are not entirely consistent [21
]. A recent study of leisure time physical activity during pregnancy is more in agreement with the results of the present study. Hegaard et al. [22
] found no association with mean birth weight or the risk of giving birth to low (<2,500
g) or high (≥4,500
g) birth weight infants. The inconsistent results in these studies could be due to different measures of physical activity, in addition to the possibility for chance findings in the smaller studies.
There is growing evidence that overweight or obesity before pregnancy is a risk factor for macrosomia [3
]. The results from the present study suggest that the effects of maternal pre-pregnancy overweight were associated with higher birth weight only among women who reported no or low level of activity. This is contradictory to the findings by Löf et al. [27
] who showed that a high pre-pregnancy activity level did not reduce the risk of high birth weight infants in women who were overweight or gained much weight in pregnancy. Nevertheless, physical activity may improve maternal weight control, both before and during pregnancy [37
The strengths of the present study include the prospective design, the large sample size of women reporting physical activity before pregnancy, and the standardized measures of size at birth obtained from the Medical Birth Registry of Norway. However, some of the categories of physical exercise (e.g., inactive) suffered from small samples size, and this could result in chance findings. Moreover, as in any observational study, residual confounding due to unmeasured (e.g., smoking during pregnancy and gestational weight gain) or unknown factors cannot be ruled out. Since the information on physical activity was self-reported, it could be subject to misclassification, although a validation study has shown acceptable agreement with objective measures [39
]. Although pre-pregnancy physical activity has been associated with physical activity level during pregnancy [23
], it has also been shown that exercise levels decline in pregnancy [24
]. Unfortunately, our data did not allow us to examine if pre-pregnancy physical exercise was related to the activity level during pregnancy.