Adriëtte Oostvogels

210 Chapter 8 Comparison with the literature This study confirms earlier reported growth patterns to overweight as found in other studies: overweight children had higher birth weight, a higher BMI peak and an earlier adiposity rebound. 6,7,12-14 Moreover, to our knowledge, this is the first study that compared longitudinal growth patterns to childhood overweight that focusses on socioeconomic and ethnic inequalities in BMI growth patterns for boys and girls separately. This study found a higher prevalence of overweight in the non-European origin children at age 5-6 years, but a similar growth pattern to overweight for children of European origin and non-European origin. Therefore, this study adds to the literature that ethnic inequalities in growth patterns, for instance accelerated growth in the first year, might simply be a reflection of the higher prevalence of overweight in these groups, 29,38-40 rather than a specific ethnic growth pattern. Regarding SES, this study also found a higher prevalence of overweight in the low SES group compared to the high SES group. In contrast to the ethnic groups, however, also the growth patterns to overweight were different for the low and high SES group. BMI was higher in the high SES group during the first two years, but lower at age 5-6 years compared to the low SES group. Similar patterns were found in studies investigating socioeconomic inequalities in growth patterns. In Generation R and ALSPAC, children in the high SES group had a higher BMI at age 2 years, 41,42 but a lower BMI at age 6 and 10 years. 41,43 This suggests that, socioeconomic inequalities in growth patterns do not simply result from socioeconomic inequalities in prevalence of overweight, but that children with a low socioeconomic background who end up being overweight, truly have a different growth pattern towards that overweight than children of a high socioeconomic background. Underlying mechanisms There are different mechanisms that could underlie the SES differences found in our cohort, but infant feeding patterns can probably be ruled out. Although children with low SES in our cohort had shorter breastfeeding duration and earlier introduction of solid foods, 44 which is associated with the development of childhood overweight, 45,46 they had lower BMIs in the first 2 years. Moreover, within our cohort, differences in feeding style did not mediate the association of low SES with weight-for-length gain from 1-5 years. 44 The lower BMI in the first 2 years might be explained by maternal smoking. Maternal smoking during pregnancy impairs foetal growth and causes low birth weight, 47-49 which often leads to catch-up growth and an increased risk of overweight in childhood. 50 In this study, mothers with a low SES were more likely to smoke during pregnancy, especially if