BMI status and weight trajectories across females' reproductive years and risk of adverse pregnancy outcomes: a prospective cohort study

Background: Prepregnancy body mass index (BMI) is a well-established risk factor of adverse pregnancy outcomes (APOs). The associations of long-term and short-term weight trajectories with APOs are less clear.

Objectives: This study aimed to determine the associations of weight trajectories during females' reproductive years, before and between pregnancies, with risk of APOs.

Methods: We followed 16,241 females (25,386 singleton pregnancies) participating in a prospective cohort, the Nurses' Health Study II. Weight at age 18 y, current weight, and height were assessed at baseline (1989), and weight was updated biennially. Pregnancy history was self-reported in 2009. The primary outcome was a composite of hypertensive disorders of pregnancy (HDP), gestational diabetes (GDM), preterm birth, and stillbirth. Secondary outcomes were individual APOs. The associations of weight change with APOs were estimated using log-binomial regression, adjusting for demographic, lifestyle, reproductive factors, and baseline BMI (in kg/m²).

Results: The mean (standard deviation [SD]) age at first in-study pregnancy was 33.7 (4.1) y. The mean (SD) time from age 18 y to pregnancy, baseline to pregnancy, and between pregnancies was 16.3 (4.0), 6.1 (3.0), and 2.9 (1.6) y, with a corresponding weight change of 6.4 (9.1), 3.1 (5.8), and 2.3 (4.8) kg, respectively. Of the pregnancies, 4628 (18.2%) were complicated by ≥1 APOs. Absolute weight change since age 18 y was most strongly associated with APOs. Compared with females whose weight remained stable (0-2 kg) since age 18, females who gained >2 kg had higher risk of APO (2.1-9.9 kg, relative risk [RR]: 1.12; 95% confidence interval [CI]: 1.02, 1.23; 10.0-14.9 kg, RR: 1.43; 95% CI: 1.29, 1.60; ≥15 kg, RR: 1.87; 95% CI: 1.69, 2.08), primarily driven by HDP and GDM. The associations of per 1 kg weight gain before and between pregnancies with HDP were nearly identical.

Conclusions: Weight trajectories prior to and between pregnancies were associated with the risk of APOs, particularly HDP. Longer periods of weight gain, corresponding to greater absolute weight gain, were most strongly associated with higher risk of APOs.