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Background <p>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.</p> Methods <p>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&#xa0;day range.</p> Results <p>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.</p> Conclusion <p>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.</p>

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Within-individual changes in BMI and menstrual irregularity: a cohort study using real-world data

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年1月28日

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  • Shiori Itoi,
  • Makiko Sampei,
  • Takayuki Tatsumi,
  • Risa Ishida,
  • Gentaro Izumi,
  • Yutaka Osuga,
  • Kaori Koga,
  • Satoshi Narumi,
  • Naho Morisaki

摘要

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.