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) 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 ethnicity×time×parity interaction (p<0.001).

Results: Primiparous South Asian women had a 1.45 (95% CI 0.39 to 2.52) kg/m² higher and Middle Eastern women had 1.43 (0.16 to 2.70) kg/m 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 more in South Asian women, 1.48 (0.31 to 2.64) kg/m more in Middle Eastern women and 2.49 (0.55 to 4.42) kg/m 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 higher in African women and 1.94 to 2.78 kg/m 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.