Abstract
BACKGROUND: The impact of body mass index (BMI) on reproductive health is well-established, but the effect of within-individual changes in BMI on the menstrual cycle, a valuable and accessible indicator of reproductive health, remains less studied. This study aimed to investigate the association between changes in individuals’ BMI and menstrual irregularity. METHODS: This cohort study analyzed data from a widely used menstrual tracking app in Japan. Menstrual cycle logs were collected from January 2019 to March 2021, and background information from questionnaires (Wave 1: Jan-Mar 2020; Wave 2: May-Jun 2020). Long-term BMI changes were defined as changes from age 18 to Wave 2, and short-term changes were defined as changes from Wave 1 to Wave 2. BMI was categorized as underweight (15–18.4), normal (18.5–22.9), and overweight/obese (23–35). Menstrual irregularity was defined as an individual’s average cycle length ranging outside the 24–38 day range. RESULTS: A total of 126,008 cycles from 5,444 individuals were analyzed. Individuals with overweight/obesity who became normal weight long-term had significantly lower odds of menstrual irregularity (long-term: aOR 0.39, 95% CI 0.20–0.75; short-term: aOR 0.61, 95% CI 0.18–2.02) compared to those who remained overweight/obese. Conversely, increases from normal weight to overweight/obese increased the odds (long-term: aOR 2.02, 95% CI 1.50–2.71; short-term: aOR 2.53, 95% CI 1.31–4.90) compared to those who remained normal weight. No significant association was found for individuals with underweight. CONCLUSION: Within-individual changes in BMI influence menstrual irregularity. Long-term, but not short-term, reductions from overweight/obese BMI to the normal BMI category were associated with lower odds of irregular cycles, whereas increases from a normal BMI to overweight/obesity raised the risk. These findings highlight the importance of sustainable weight management in enhancing reproductive health. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12905-026-04308-2.