Ethnic differences in body mass index trajectories from 18 years to postpartum in a population-based cohort of pregnant women in Norway.

Objectives To explore ethnic differences in changes in body mass index (BMI) from the age of 18 years to 3 months postpartum. Design A population-based cohort study. Setting Child Health Clinics in Oslo, Norway. Participants Participants were 811 pregnant women (mean age 30 years). Ethnicity was categorised into six groups. Primary outcome measures The outcome variable was BMI (kg/m(2)) measured at the age of 18 and 25 years, at prepregnancy and at 3 months postpartum. Body weight at 18 years, 25 years and prepregnancy were self-reported in early pregnancy, while body height and weight at 3 months postpartum were measured. The main statistical method was generalised estimating equations, adjusted for age. The analyses were stratified by parity due to ethnicityxtimexparity interaction (p<0.001). Results Primiparous South Asian women had a 1.45 (95% CI 0.39 to 2.52) kg/m (2) higher and Middle Eastern women had 1.43 (0.16 to 2.70) kg/m(2) higher mean BMI increase from 18 years to postpartum than Western European women. Among multiparous women, the mean BMI increased 1.99 (1.02 to 2.95) kg/m(2) more in South Asian women, 1.48 (0.31 to 2.64) kg/m(2) more in Middle Eastern women and 2.49 (0.55 to 4.42) kg/m(2) more in African women than in Western European women from 18 years to prepregnancy. From 18 years to postpartum, the mean increase was 4.40 (2.38 to 6.42) kg/m(2) higher in African women and 1.94 to 2.78 kg/m(2) higher in the other groups than in Western European women. Conclusions Multiparous women of ethnic minority origin seem substantially more prone to long-term weight gain than multiparous Western European women in Norway.