Association of Fecundity With Changes in Adult Female Weight

Abstract

Objective: To evaluate whether weight change since age 18 years, current body mass index (BMI), and BMI at age 18 years are associated with fecundity.

Methods: Our study included 1,950 women in the Nurses' Health Study 3 (2010-2014), a prospective cohort study, currently attempting pregnancy. Height, current weight, and weight at age 18 years were self-reported on the baseline questionnaire. Every 3-6 months thereafter, women reported the current duration of their pregnancy attempt. Multivariable-accelerated failure time models were used to estimate the time ratios and 95% confidence intervals (CIs).

Results: For every 5-kg increase in body weight from age 18 years, current duration of pregnancy attempt increased by 5% (95% CI 3-7%). Compared with women who maintained weight, the adjusted median current duration was 0.5 months shorter in those who lost weight, 0.3 months longer for those who gained 4-9.9 kg and 10-19.9 kg, and 1.4 months longer for those who gained 20 kg or more (P trend ≤.001). The adjusted time ratio (95% CI) for a 5-kg/m increase in current BMI was 1.08 (1.04-1.12). After multivariable adjustment (including adjustment for current BMI), being underweight at age 18 years (BMI less than 18.5) was associated with a longer current duration of pregnancy attempt compared with normal-weight women (time ratio 1.25, 95% CI 1.07-1.47); however, being overweight or obese at age 18 years was not associated with fecundity.

Conclusion: Gaining weight in adulthood, being overweight or obese in adulthood, and being underweight at age 18 years were associated with a modest reduction in fecundity.

Level of evidence: II.

Figure 1

Flow diagram explaining the final cohort of women in the analysis.

Figure 2

Association between weight change since age 18 years (A), current body mass index (BMI) (B), and BMI at age 18 years (C) and current duration of pregnancy attempt. Accelerated failure time models were used to estimate the time ratios and 95% confidence intervals (CI). All models are adjusted for age (years), race (white compared with other), smoking status (never, former, current, or missing), and marital status (married or not married). Weight change models were further adjusted for BMI at age 18 years. *P<.05 for the comparison of a time ratio for a specific exposure group compared to the reference group.

Figure 3

Association between changes in body mass index (BMI) category between age 18 years and present and current duration of pregnancy attempt. Accelerated failure time models were used to estimate the time ratios and 95% confidence intervals (CI). All models are adjusted for age (years), race (white compared with other), smoking status (never, former, current, or missing), and marital status (married or not married). The following groups were not presented in the figure due to low numbers: women with a BMI of 18.5–24.9 kg/m² at age 18 years and a BMI of <18.5 kg/m² in adulthood (n=19) and women with a BMI of ≥25 kg/m² at age 18 years and a BMI of <18.5 kg/m² in adulthood (n=0). *P<.05 for the comparison of a time ratio for a specific exposure group compared to the reference group.