Do physical activity, sitting time and body mass index affect fertility over a 15-year period in women? Data from a large population-based cohort study

STUDY QUESTION: Do physical activity (PA), sitting time (ST) and body mass index (BMI) affect fertility over a 15 year period in Australian women? SUMMARY ANSWER: Moderate and high levels of PA confer advantages for fertility in women with normal BMI, but increased risk of infertility was observed in obese women. WHAT IS KNOWN ALREADY: Higher BMI is positively associated with higher rates of problems with fertility, but the effects of physical activity and sitting time on fertility are less well understood. STUDY DESIGN, SIZE, DURATION: Participants in The Australian Longitudinal Study of Women's Health (ALSWH) completed mailed surveys in 2000, with follow-ups in 2003, 2006, 2009, 2012 and 2015 (N = 6130). PARTICIPANTS/MATERIALS, SETTING, METHODS: Participants were aged 22 to 27 in 2000. They were asked to report their physical activity levels, sitting time and fertility problems in each survey from 2000 to 2015. BMI was calculated from self-reported weight and height. Cumulative incidence of fertility problems was calculated from 2000 to 2015 and hazard ratios (HR) and 95% Cls were calculated using survival analysis. MAIN RESULTS AND THE ROLE OF CHANCE: From 2000 to 2015, the cumulative incidence of fertility problems was 15.4% (95% CI: 14.5-16.4). High levels of PA were associated with reduced risk of problems with fertility [HR 0.82 (95% CI: 0.69-0.98)], and higher BMI was positively associated with fertility problems [overweight: HR 1.18, (95% CI 0.99-1.39); obese: HR 1.36, (95% CI 1.14-1.63)]. In survival analyses, incidence rates were highest in every survey interval in women who reported low PA levels and in women who were obese. Overall, ST was not associated with fertility problems. In stratified models, high levels of PA attenuated the risk of problems with fertility in women who were in the normal BMI category [HR 0.64, (95% CI 0.49-0.82)]. LIMITATIONS, REASONS FOR CAUTION: The ALSWH relies on self-reported data, which may be subject to recall bias. WIDER IMPLICATIONS OF THE FINDINGS: The study provides estimates of problems with fertility in a cohort of young adult Australian women, and the results indicate that these are inversely associated with physical activity levels and positively associated with BMI. However, the high infertility risk in obese women was not attenuated by high levels of PA. The protective effects of PA were only observed in women with normal BMI. As rates of developing problems with fertility were highest in every survey interval among women who reported low levels of physical activity and in women who were obese, these findings suggest that improving physical activity levels could be an affordable strategy to reduce problems with fertility in women who are trying to conceive. These findings should be considered by clinical and public health practitioners.