Adverse pregnancy outcomes attributable to overweight and obesity across maternal birth regions: a Swedish population-based cohort study.

Background: Whether there are differences in the contribution of overweight and obesity to adverse pregnancy outcomes between migrant and non-migrant women in high-income countries, which might increase health inequalities, remains unclear. Therefore, in this study, we aimed to estimate the contribution (including the proportion and number of attributable cases) of overweight and obesity to a wide range of adverse pregnancy outcomes in Swedish-born and migrant women.

Methods: This population-based cohort study used nationwide population registries in Sweden. All outcomes and covariates were collected from the Medical Birth Register (delivery and maternal characteristics), National Patient Register (inpatient and specialised outpatient care), the Cause of Death Register (all deaths in Sweden), the Longitudinal Integrated Database for Health Insurance and Labour Market Studies (socioeconomic data), and the Total Population Register (maternal birth country data). Women with missing records of BMI at the first antenatal visit, country of birth, or covariates, were excluded from the study. BMI was measured during the first antenatal visit. Maternal country of birth was categorised into Sweden and seven super-regions. The proportion (ie, population attributable fractions [PAFs]) and the number of adverse pregnancy outcomes attributable to overweight and obesity were calculated, adjusting for maternal age, gestational age at first antenatal visit, maternal parity, smoking status, maternal somatic conditions, child's sex, socioeconomic and demographic variables.

Findings: We identified 2 228 416 singleton pregnancies between Jan 1, 2000, and Dec 31, 2020 of 1 245 273 women. 254 778 (11·4%) pregnancies with missing records of BMI at the first antenatal visit, country of birth, or covariates were excluded, which resulted in a final analytical cohort of 1 973 638 pregnancies carried by 1 164 783 women. The overall mean maternal age of the study population was 30·8 years (SD 5·1). As estimated by PAFs, overweight and obesity contributed to a large proportion of adverse pregnancy outcomes: gestational diabetes (52·1% [95% CI 51·0-53·2]), large-for-gestational age (36·9% [36·2-37·6]), pre-eclampsia (26·5% [25·7-27·3]), low Apgar score (14·7% [13·5-15·9]), infant mortality (12·7% [9·8-15·7]), severe maternal morbidity (henceforth referred to as a near-miss event; 8·5% [6·0-11·0]), and preterm birth (5·0% [4·4-5·7]) in the total study population. PAFs varied between maternal birth regions.

Interpretation: Interventions to reduce overweight and obesity have the potential to mitigate the burden of adverse pregnancy outcomes and possibly reduce inequalities in reproductive health. Therefore, public health practice and policy should prioritise efforts to prevent overweight and obesity among women of childbearing age.

Funding: Swedish Research Council.