Maternal overweight and obesity in early pregnancy and risk of infant mortality: a population based cohort study in SwedenBMJ 2014; 349 doi: https://doi.org/10.1136/bmj.g6572 (Published 02 December 2014) Cite this as: BMJ 2014;349:g6572
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Re: Maternal overweight and obesity in early pregnancy and risk of infant mortality: a population based cohort study in Sweden
We read with interest the excellent article by Johansson et al. (1) underlying the prospective association of maternal overweight and obesity in early pregnancy with the risk of infant mortality. The authors observed that the risk of infant mortality increased with maternal body mass index (BMI) among women who were overweight and obese at early pregnancy. Their contribution must be recognized since they overcome limitations of previous studies and present extensive data from a large cohort of Swedish pregnant women. However, the potential modulatory effect of pre- and early pregnancy physical activity was not considered in this study and might have played a partial role in the study results.
Emerging evidence suggests that being physically active before and during pregnancy may improve glucose levels (2) and prevent excessive maternal weight gain (3, 4), gestational diabetes mellitus (GDM) (4-6), preeclampsia (5, 7), and abnormal fetal growth (8). Conversely, the general tendency to reduce physical activity levels during pregnancy (9-11) is associated with increased risk for excessive weight gain and developing GDM or preeclampsia (5, 6).
Congenital anomalies, the major cause of infant mortality in live singleton term births in this cohort (accounting for 48.4% of the deaths) (1), are caused by a number of factors among which GDM plays a relevant role (12). Maternal obesity is highly associated with GDM, which increases perinatal mortality risk. In obese women with GDM, a reduced (<5 kg) gestational weight gain has shown to be associated with better obstetric and neonatal outcomes than excessive weight gain (13). In this line, pre- or during pregnancy physical activity may be a protecting factor since it has been shown to prevent excessive maternal weight gain (3, 4) and GDM (4, 6). In addition, preterm birth, a well known risk factor for neonatal mortality (14), accounted for 48% of all deaths in the current study. It is well known that pre-eclampsia, which is promoted by obesity and the leading cause of perinatal morbidity and mortality in the developed world (15), is highly associated with preterm birth (16). Evidence suggests that both preterm birth and pre-eclampsia may be improved with regular physical activity (7, 17). A potential mechanism for the effect of physical activity could be through a better configuration of body weight and composition before and during pregnancy. Therefore, the possibility that physical activity interacts with BMI on infant mortality should not be discarded. In fact, Sorbye et al. (18) revealed that the lowest perinatal mortality in gestants with pre-pregnant overweight in comparison with normal weight was seen in those who performed recreational physical activity at least once a week. Their results were, however, contrary for women with pre-pregnant BMI>30, where the highest risk was observed in those involved in recreational physical activity. Although their results are still preliminary and must be replicated accounting for adherence to physical activity recommendations during pregnancy and type of exercise (19), they observed a significant interaction between BMI and physical activity on mortality that must be acknowledged.
Although it is unlikely that including physical activity in the models may significantly change the conclusions of the study by Johansson et al. (1), it is possible that overweight and/or obese pregnant women who engaged in recommended levels of physical activity during pre- or early pregnancy (at least 150 min/week of moderate physical activity (19)) could present differential infant mortality rates than those who spent most of their time in sedentary behaviors. We suggest that future population-based studies assessing the risk of maternal exposures on infant mortality may benefit from collecting data on pre- or early pregnancy physical activity levels.
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19. U.S. Department of Health and Human Services. 2008 physical activity guidelines for Americans. Washington (DC): Department of Health and Human Services; 2008. p. 41.
Competing interests: No competing interests