Being underweight or obese in adolescence may increase the risk of female infertility later in life: The Japan Nurses' Health Study

Objectives: Underweight or obesity is associated with infertility in women, but large cohort studies with a life course perspective are rare. We explored the association between body mass index (BMI) at age 18 and subsequent infertility among Japanese women. Study design: In total, 15,907 married women aged 30–44 years who participated in a prospective cohort baseline survey during 2001–2007 were cross-sectionally analyzed. Those with a history of cancer, and those with a partner with infertility or missing data were excluded. All data were obtained from self-reported questionnaires. Main outcome measures: The main outcome measure was infertility, defined as a self-reported history of infertility for at least 2 years. Age and cause of infertility were also recorded. Results: Overall, 2825 participants (17.8 %) reported being infertile. The proportions of women who were underweight (BMI <18.5 kg/m²), overweight (BMI ≥25 and < 30 kg/m²), and obese (BMI ≥ 30 kg/m²) at age 18 were 12.8 %, 4.5 %, and 0.4 %, respectively. Multiple logistic regression analysis revealed a 1.31-fold and a 1.82-fold increase in the rate of infertility in those women who were underweight or obese during adolescence, respectively, compared with those of normal weight. Further analysis by cause of infertility revealed no significant associations with overweight or obesity at age 18, but being underweight was positively associated with infertility of unknown cause. Conclusions: Being underweight or obese at adolescence is a potential risk factor for subsequent infertility among Japanese women. In particular, being underweight during adolescence may be a contributing factor to unexplained infertility later in life.