Weight change between successive pregnancies and risks of stillbirth and infant mortality: a nationwide cohort study

Background: Maternal overweight and obesity are risk factors for stillbirth and infant mortality. Whether temporal changes in maternal weight affect these risks is not clear. We aimed to assess whether change of BMI between first and second pregnancies affects risks of stillbirth and infant mortality in the second-born offspring.

Methods: In a Swedish population-based cohort of women who gave birth to their first and second child between Jan 1, 1992, and Dec 31, 2012, we investigated associations between change in maternal body-mass index (BMI) during early pregnancy from first to second pregnancies and risks of stillbirth and neonatal, postneonatal, and infant mortality after the second pregnancy. Relative risks (RRs) for each outcome according to BMI change categories were calculated with binomial regression.

Findings: Complete information was available for 456,711 (77.7%) of 587,710 women who had their first and second single births in the study period. Compared with women with a stable BMI (change between -1 kg/m(2) and <1 kg/m(2)) between pregnancies, the adjusted RRs for women who gained at least 4 BMI units between pregnancies were 1.55 (95% CI 1.23-1.96) for stillbirth and 1.29 (1.00-1.67) for infant mortality. Stillbirth risks increased linearly with increased BMI gain. Risks of infant mortality in second pregnancy only increased with BMI gain in women with healthy BMI (<25 kg/m(2)) during first pregnancy; the adjusted RR for healthy weight women who gained 2 to less than 4 BMI units was 1.27 (1.01-1.59) and for those who gained 4 BMI units or more the adjusted RR was 1.60 (1.16-2.22). In overweight women (BMI ≥25 kg/m(2)), weight loss before pregnancy reduced risk of neonatal mortality.

Interpretation: Our findings emphasise the need to prevent weight gain before pregnancy in healthy and overweight women and that weight loss should be promoted in overweight women.

Funding: Swedish Research Council for Health, Working Life and Welfare, and Karolinska Institutet.