Socioeconomic disparities in birth weight and body mass index during infancy through age 7years: a study within the Danish National Birth Cohort

Background Socioeconomic inequalities in birth weight and in body mass index (BMI) later in childhood are in opposite directions, which raises questions about when during childhood the change in direction happens. We examined how maternal and paternal education and household income were associated with birthweight z-scores and with BMI z-scores at age 5 and 12months and 7years, and we examined the socioeconomic differences in the tracking of these z-scores across infancy and childhood. Methods The associations were studied in a cohort of children in the Danish National Birth Cohort, single born between 1997 and 2003, for whom information on body size from at least 1 of 4 time points (n=85062) was recorded. We examined the associations using linear mixed-effects modelling. Results Children from families with a low maternal and paternal educational level changed their body size z-scores upwards between birth and age 7years. At age 5 and 12months, there were no educational gradient. A low maternal educational level was associated with lower birth weight for gestational age z-scores at birth for boys (-0.199; 95% CI -0.230 to -0.169) and girls (-0.198; 95% CI -0.229 to -0.167) and higher BMI z-scores at age 7 for boys (0.198; 95% CI 0.154 to 0.242) and girls (0.218; 95% CI 0.173 to 0.264). There was not a similarly clear pattern in the tracking between different household income groups. However, a low household income level was associated with higher z-scores of both birth weight and BMI at age 7years, but with a much weaker gradient at 5 and 12months. Conclusions The educational gradient shifts from positive with birth weight, to none during infancy to inverse with BMI at age 7years. In contrast, the income gradient was positive at birth and at 7years and much weaker during infancy.