Adriëtte Oostvogels

8 211 Growth patterns to overweight their children were overweight at age 5-6 years.(Supplementary Table 5) A previous study within our cohort showed that maternal smoking during pregnancy largely attributed to the higher prevalence of infants born small-for-gestational age in the low SES group. 51 Therefore, the socioeconomic differences in growth patterns to overweight observed in infancy could be explained by maternal smoking during pregnancy. Furthermore, in the earlier mentioned study on the association between SES and weight-for-length gain from 1-5 years, the most important mediator was maternal prepregnancy BMI. 44 The increased growth after two years might therefore be a consequence of maternal overweight, as prevalence of maternal overweight is higher in women of low SES. 52,53 Maternal prepregnancy BMI is positively associated with childhood BMI, which could result in higher BMI in children of the low SES group. 54,55 This could be the consequence of an unhealthier lifestyle. Important childhood mediators in the inverse association between SES and childhood overweight at age 5 years are maternal TV watching and consuming breakfast and TV watching by the child. 56 More in general, not participating in organized sport was found to be associated with increased BMI z-scores, 57 and overweight children from a low socioeconomic background sleep shorter than their normal weight peers. 58,59 In our study children in the low SES group had more often mothers with overweight, they watched more TV and slept less, and were less often a member of a sports club. (Supplementary Table 5) Hence, the accelerated growth of the low SES group might be the consequence of an unhealthier lifestyle. Implications This study showed that children with overweight from the low/middle SES group have lower BMIs in the first year compared to the high SES group, but after 2 years, these groups accelerated in growth and had a higher BMI at age 5-6 years. Therefore, it is important to closely monitor children with a low SES background as their weight might be in the normal range before the age of 2, but this does not prevent them from being at risk of developing overweight later on. An early adiposity rebound should be an indicator for extra care, but to identify the adiposity rebound, it is necessary to measure anthropometrics frequently, for instance every 3-4 months, in both infancy and early childhood. This is of foremost importance as BMI in childhood is a predictor of adult BMI 3-5 and childhood SES has a greater influence on adult BMI and the prevalence of overweight/obesity than adult SES. 60 A combination of both a low SES and high BMI in early childhood might therefore be most detrimental for adult BMI.