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Maintaining a Standard BMI Leads to a Stable Menstrual Cycle

Published Jul 8, 2026 7:00 PM ・ Updated Jul 9, 2026 1:19 AM ・ 25 min read ・ Source: PR TIMES

A research group comprising Naho Morisaki and Shiori Itoi from the Center for Women's Health at the National Center for Child Health and Development (Location: Setagaya-ku, Okura, Tokyo; Director: Takashi Igarashi), and Kaori Koga from Chiba University, along with MTi Inc. (Location: Shinjuku-ku, Tokyo; President and CEO: Toshihiro Maeta), the operator of the menstrual management app 'Lunaring', conducted a study using large-scale cohort data from 'Lunaring' users. They examined the relationship between Body Mass Index (BMI) and menstrual cycle regularity from the perspectives of 'current BMI' and 'changes in BMI within the same individual'. The analysis revealed that both the average cycle length and individual variation (standard deviation of cycle length) were most stable around a standard BMI. A correlation was observed where instability tended to increase when BMI was either lower or higher than the standard range. Furthermore, focusing on changes in body composition, individuals who transitioned from a standard BMI to overweight/obesity showed an increase in menstrual irregularities, while those who transitioned from overweight/obesity to a standard BMI showed a decrease in menstrual irregularities. These findings suggest that maintaining a BMI around the standard range long-term (or moving closer to a standard BMI) is desirable from the perspective of menstrual cycle regularity.

These research findings were published in the international academic journals npj Women’s Health and BMC Women’s Health in February 2025 and January 2026, respectively.

[Figure 1: Menstrual Cycle is Most Stable Around Standard BMI (Schematic Diagram)]

[Key Points of the Press Release]

Using prospective log data from a menstrual management app (up to approximately 190,000 cycles), it was found that both average cycle length and individual variation are most stable around a standard BMI, and tend to be unstable when below or above the standard range. It was also confirmed that compared to a standard BMI, lower or higher BMIs are associated with longer menstrual cycles and greater individual variation.

Amenorrhea and oligomenorrhea [1], indicators of menstrual abnormalities, also increased as BMI deviated from the standard range. Low weight (BMI 15–18.4) was associated with amenorrhea, and obesity (BMI 23–35) was associated with both amenorrhea and oligomenorrhea (all compared to standard BMI).

Analysis of BMI category changes within the same individual showed a tendency for menstrual irregularities to occur more easily when transitioning from a standard BMI to overweight/obesity, and a tendency for menstrual irregularities to occur less easily when transitioning from overweight/obesity to a standard BMI.

These results suggest that both the 'extremes of BMI (underweight/obesity)' and 'changes in body composition within the same individual' may be related to the regularity of the menstrual cycle (a 'sign of health' observable in daily life).

[1] Oligomenorrhea is a type of menstrual irregularity where the menstrual cycle occurs at longer intervals than usual, specifically 39 days or more, or less than three times a year.

[Background and Objectives]

The menstrual cycle is one of the 'health indicators' that can reflect fertility and the state of the endocrine system. While it is known that BMI affects ovulation and menstruation, research that has extensively evaluated BMI and quantitative indicators of the menstrual cycle (average cycle length and individual variation) in the general Asian population has been limited. Furthermore, there has been insufficient data on how menstrual irregularities change when BMI changes within the same individual. Therefore, this research group aimed to examine the relationship between menstrual cycle regularity and BMI from both 'current BMI value' and 'BMI changes' using large-scale cohort data from menstrual management app users.

[Research Overview] This presentation integrates the findings from the following two studies using the same menstrual record app data:

Study Design and Participants

Menstrual logs recorded from January 2019 to March 2021 and in-app questionnaire data from users of 'Lunaring', a widely used menstrual management app in Japan, were linked and analyzed. Questionnaires were primarily used from Wave 1 (January–March 2020) and Wave 2 (May–June 2020).

Study 1: Relationship between BMI (current value) and Menstrual Cycle/Abnormalities (Published in npj Women’s Health)

191,426 menstrual logs from 8,745 individuals were analyzed to evaluate the relationship between BMI and cycle length/variation, amenorrhea, and oligomenorrhea. Basal body temperature data from 15,883 cycles of 3,221 individuals were analyzed.

BMI was classified into underweight (15–18.4), normal (18.5–22.9), overweight (23–24.9), and obese (25–35).

Outcome Measures:

· Average cycle length

· Individual variation (standard deviation of cycle length for each individual)

· Indicators of menstrual abnormalities such as amenorrhea and oligomenorrhea

· Proportion of biphasic cycles [2] (indicator of ovulation)

[2] A biphasic cycle refers to a state where basal body temperature clearly divides into a 'low temperature phase (follicular phase)' and a 'high temperature phase (luteal phase)'.

Study 2: Relationship between Changes in BMI Category and Menstrual Irregularity (Published in BMC Women’s Health)

126,008 cycles from 5,444 individuals were analyzed to evaluate the relationship between BMI category changes (long-term changes) from age 18 to the Wave 2 time point, and BMI category changes (short-term changes) over several months between Wave 1 and Wave 2, and menstrual irregularity. Menstrual irregularity was defined as an average cycle length outside the range of 24–38 days.

BMI was classified into underweight (15–18.4), normal (18.5–22.9), and overweight/obese (23–35).

Exposure (Changes in BMI Category):

1) Long-term change: Change in BMI category from age 18 to the time of the survey

2) Short-term change: Change in BMI category over several months during the survey period

Outcome Measures:

Proportion of menstrual abnormalities (average cycle length outside the range of 24–38 days for individuals)

Main Research Findings

1) BMI (current value) and Cycle Length/Variation: J-shaped relationship with the lowest point around normal BMI

Average cycle length was shortest in the normal BMI range, and tended to lengthen when BMI was lower or higher than the normal range (Figure 1).

Variation in cycle length (standard deviation of average cycle length) was also minimal in the normal BMI range, and tended to increase when BMI was lower or higher than the normal range.

2) Amenorrhea and Oligomenorrhea: Tendency to increase as BMI deviates from standard

Analysis by BMI classification showed, with standard BMI as the reference:

・Amenorrhea: Tended to increase in the underweight and obese groups.

・Oligomenorrhea: Tended to increase in the overweight and obese groups.

Analysis using continuous BMI values showed a tendency for both amenorrhea and oligomenorrhea to increase with lower or higher BMIs than the normal range, with normal BMI values as the low point.

3) Indicator of Ovulation (Basal Body Temperature Biphasic Pattern): Higher in standard BMI, tending to decrease in low/high BMI

The proportion of biphasic cycles, estimated from basal body temperature, showed a tendency to be higher in standard BMI and decrease in low/high BMI.

4) Within-individual BMI Changes: Irregularities increase with upward changes, and decrease with long-term normalization (Figure 2)

Regarding the changes in BMI category from age 18 to the time of the survey and menstrual irregularity at the time of the survey, the following associations were observed:

・Long-term transition from standard BMI to overweight/obesity tended to be associated with an increase in menstrual irregularities.

・Long-term transition from overweight/obesity to standard BMI tended to be associated with a decrease in menstrual irregularities.

Furthermore, regarding short-term BMI category changes within the survey period, the following associations were observed:

・Short-term transition from standard BMI to overweight/obesity tended to be associated with an increase in newly occurring menstrual irregularities during the follow-up period.

[Figure 2: Relationship between Changes in BMI Category and Menstrual Irregularity (Adjusted Odds Ratio)]

[Comments from Presenters]

This study, using large-scale data from a menstrual management app, demonstrated that as BMI deviates from the standard range, menstrual cycle extension and increased variation, as well as menstrual abnormalities, increase. It also showed that changes in BMI category within the same individual are associated with the risk of menstrual abnormalities.

While this is an observational study and does not establish causality, continuous monitoring of the menstrual cycle may help to 'visualize' 'health changes' associated with changes in body composition.

Future challenges include integrating more detailed information on nutrition, physical activity, and obstetric history to elucidate the mechanisms of menstrual cycle changes and evaluate the effectiveness of supportive interventions (including digital interventions).

[Publication Information]

<1>

Title: Body mass index and menstrual irregularity in a prospective cohort study of smartphone application users.

Authors: Shiori Itoi1,2,3, Makiko Sannomiya1,4, Takayuki Tatsumi2,5, Yutaka Osuga3,6,7, Kaori Koga3,8,

Satoshi Narumi9,10, Naho Morisaki1,11

Affiliations:

1) Department of Social Medicine, National Center for Child Health and Development

2) Department of Reproductive Medicine, Center for Women's Health, National Center for Child Health and Development

3) Department of Obstetrics and Gynecology, Graduate School of Medicine, The University of Tokyo

4) Department of Health and Sport Sciences, Nippon Sport Science University

5) Umegaoka Clinic for Obstetrics and Gynecology

6) Clinical Research Center, Teikyo University

7) Department of Obstetrics and Gynecology, School of Medicine, Teikyo University

8) Department of Obstetrics and Gynecology, Graduate School of Medicine, Chiba University

9) Department of Molecular Endocrinology, National Center for Child Health and Development

10) Department of Pediatrics, School of Medicine, Keio University

11) Division of Health Promotion, Center for Women's Health, National Center for Child Health and Development

Journal: npj Women’s Health Publication Date: February 27, 2025

DOI: 10.1038/s44294-025-00065-z

<2>

Title: Within-Individual Changes in BMI and Menstrual Irregularity: A Cohort Study Using Real-World Data.

Authors: Shiori Itoi1,2,3, Makiko Sannomiya1,4, Takayuki Tatsumi2,5, Risa Ishida3, Gentaro Izumi3, Yutaka Osuga3,6,7, Kaori Koga3,8, Satoshi Narumi9,10, Naho Morisaki1,11