The increasing prevalence of overweight and obesity among young women in most countries1–5 underlines its large potential impact on adverse pregnancy outcome. During pregnancy, maternal overweight/obesity increases the risk of serious complications, such as gestational diabetes, hypertension and pre-eclampsia, which lead to increased risk for preterm birth (PTB), reduced or augmented fetal growth and stillbirth.6–8 As the obesity epidemic evolves, it is important to monitor the extent to which overweight/obesity contributes to the occurrence of these adverse pregnancy outcomes.
Population attributive fraction (PAF) calculations have been used to measure the proportion of the adverse outcome that is attributable to the occurrence of the risk factor in the population. When applied to smoking, the PAF revealed a substantial impact of smoking on the occurrence of adverse pregnancy outcomes in most Western populations.9–11 Recognition of this large impact has led to increasing awareness among the medical profession and pregnant women for the need to prevent smoking during pregnancy. In 2002, Cnattingius et al.12 warned that overweight and obesity, because of their increasing prevalence, could parallel smoking as a contributor to the occurrence of adverse pregnancy outcomes. However, no quantitative comparisons have yet been published.
The contribution of maternal overweight/obesity to adverse pregnancy outcomes has been examined in very few populations. Three studies have focused on the contribution of maternal obesity to the occurrence of caesarean section, pregnancy-induced hypertension, and large-for-gestational-age (LGA) infants.2,13,14 Lu et al.2 showed that in Alabama (USA) the PAF for LGA infants increased from 6.5% in 1980–84 to 19.1% in 1995–99, illustrating the rising public health impact of obesity for the health of newborns. No comparable estimates have been published for Western European countries.
The prevalence of overweight/obesity in Europe is higher in immigrant groups than in native groups in general,15–17 especially among pregnant women.3 Moreover, the effect of maternal overweight/obesity on adverse pregnancy outcomes is suggested to be stronger in some non-Western immigrant groups.18,19 Hence, obesity/overweight might contribute particularly to the occurrence of adverse pregnancy outcomes among newborn children in these groups. To our knowledge, no study has investigated ethnic differences in the contribution of pre-pregnancy overweight and obesity to major pregnancy outcomes.
The aim of this study is to evaluate the contribution of women’s pre-pregnancy overweight/obesity to adverse pregnancy outcomes (small-for-gestational-age [SGA] infants, LGA infants, PTB and extreme PTB) in a multi-ethnic population. The PAF approach is applied to different ethnic groups, taking into account ethnic differences in overweight/obesity prevalence and in the associations between overweight/obesity and pregnancy outcomes. The second aim of this study is to assess whether the magnitude of these contributions varies between ethnic groups. Finally, the PAFs for overweight/obesity are compared with those of the well-documented risk factor ‘smoking’. All estimates are based on a large prospective community-based birth cohort in Amsterdam.