News | Study Finds BMI May Affect Menstrual Cycles, with BMI 20 Potentially Linked to Optimal Ovulation
Recent research found a significant association between women's body mass index (BMI) and menstrual-cycle regularity. Maintaining a BMI around 20 may be an optimal range for ovulation and fertility.
Published in npj Women’s Health, the study analyzed real-world data from more than 8,700 women in Japan using the LunaLuna menstrual-tracking app. It covered 191,426 complete menstrual cycles, making it one of the largest studies of its kind.
Menstrual Cycles Become Less Regular as BMI Moves Up or Down
The menstrual cycle is considered an indicator of women's health. Its regularity depends on stability of the hypothalamic-pituitary-ovarian (HPO) axis. A BMI that is either too high or too low may disrupt this system, causing anovulation, amenorrhea, or irregular cycles.
The study found a significantly higher risk of amenorrhea among women with a BMI below 19 or above 26. Participants with a BMI of 20 had the shortest and most regular average cycles.
Study Methods and Participants
The study used menstrual data collected through the LunaLuna app from January 2019 to March 2021. Participants completed two surveys covering health, lifestyle, education, and employment.
Of 10,465 users who completed both surveys, researchers excluded those with an extreme BMI (<15 or >35), pregnant women, those receiving infertility treatment, users of hormonal contraception or intrauterine devices, and those recording fewer than three cycles. The final analysis included 8,745 women and 191,426 cycles.
Another 3,221 women provided basal body temperature (BBT) data to assess ovulation. A luteal-phase average temperature 0.3°C higher than the follicular-phase average was classified as a biphasic cycle, indicating ovulation.
Key Finding: Reproductive Indicators Were Best at BMI 20
The team used restricted cubic spline models to analyze cycle length (CL), amenorrhea (AMB), infrequent menstruation (IMB), and the proportion of biphasic cycles.
Results showed:
Women with a BMI ≤16 or ≥30 had menstrual cycles averaging 1.03–1.06 days longer than women with a BMI of 20;
Amenorrhea risk was significantly higher with a BMI ≤19 or ≥26;
Compared with women in the normal BMI range, the likelihood of infrequent menstruation (cycles ≥39 but <90 days) was 56% higher among women who were overweight and 163% higher among those with obesity;
The proportion of biphasic cycles followed an inverted J-shaped relationship: it was highest at BMI 20, while ovulatory function gradually declined above or below that level.
In short, both low and high body weight were strongly associated with ovulatory dysfunction, which may affect fertility.
Expert Note: Small BMI Differences May Have Significant Effects
The data were limited to Japan. Because the relationship between body fat and BMI differs among populations, the team noted that the findings require further validation before being generalized worldwide.
The study also relied on self-reported BMI, creating some risk of bias, and app users may differ from the general population. Future research should account for these limitations.
Even so, the overall conclusion was clear: small departures from the observed BMI range may significantly increase the risk of irregular menstruation and anovulation. For women seeking to support fertility, a BMI around 20 may be an important optimal point.
News | Study Finds BMI May Affect Menstrual Cycles, with BMI 20 Potentially Linked to Optimal Ovulation
News | Study Finds BMI May Affect Menstrual Cycles, with BMI 20 Potentially Linked to Optimal Ovulation
Recent research found a significant association between women's body mass index (BMI) and menstrual-cycle regularity. Maintaining a BMI around 20 may be an optimal range for ovulation and fertility.
Published in npj Women’s Health, the study analyzed real-world data from more than 8,700 women in Japan using the LunaLuna menstrual-tracking app. It covered 191,426 complete menstrual cycles, making it one of the largest studies of its kind.
Menstrual Cycles Become Less Regular as BMI Moves Up or Down
The menstrual cycle is considered an indicator of women's health. Its regularity depends on stability of the hypothalamic-pituitary-ovarian (HPO) axis. A BMI that is either too high or too low may disrupt this system, causing anovulation, amenorrhea, or irregular cycles.
The study found a significantly higher risk of amenorrhea among women with a BMI below 19 or above 26. Participants with a BMI of 20 had the shortest and most regular average cycles.
Study Methods and Participants
The study used menstrual data collected through the LunaLuna app from January 2019 to March 2021. Participants completed two surveys covering health, lifestyle, education, and employment.
Of 10,465 users who completed both surveys, researchers excluded those with an extreme BMI (<15 or >35), pregnant women, those receiving infertility treatment, users of hormonal contraception or intrauterine devices, and those recording fewer than three cycles. The final analysis included 8,745 women and 191,426 cycles.
Another 3,221 women provided basal body temperature (BBT) data to assess ovulation. A luteal-phase average temperature 0.3°C higher than the follicular-phase average was classified as a biphasic cycle, indicating ovulation.
Key Finding: Reproductive Indicators Were Best at BMI 20
The team used restricted cubic spline models to analyze cycle length (CL), amenorrhea (AMB), infrequent menstruation (IMB), and the proportion of biphasic cycles.
Results showed:
Women with a BMI ≤16 or ≥30 had menstrual cycles averaging 1.03–1.06 days longer than women with a BMI of 20;
Amenorrhea risk was significantly higher with a BMI ≤19 or ≥26;
Compared with women in the normal BMI range, the likelihood of infrequent menstruation (cycles ≥39 but <90 days) was 56% higher among women who were overweight and 163% higher among those with obesity;
The proportion of biphasic cycles followed an inverted J-shaped relationship: it was highest at BMI 20, while ovulatory function gradually declined above or below that level.
In short, both low and high body weight were strongly associated with ovulatory dysfunction, which may affect fertility.
Expert Note: Small BMI Differences May Have Significant Effects
The data were limited to Japan. Because the relationship between body fat and BMI differs among populations, the team noted that the findings require further validation before being generalized worldwide.
The study also relied on self-reported BMI, creating some risk of bias, and app users may differ from the general population. Future research should account for these limitations.
Even so, the overall conclusion was clear: small departures from the observed BMI range may significantly increase the risk of irregular menstruation and anovulation. For women seeking to support fertility, a BMI around 20 may be an important optimal point.
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