Normal menstrual flow volume range and its characteristics measured from sanitary napkins in Japanese women: Discrepancy between measured and subjective menstrual flow volume.

Aim: The 2018 International Federation of Gynecology and Obstetrics abnormal uterine bleeding System-1 defines menstrual flow volume on a 3-point scale based on subjective assessment. However, the normal flow range has not been established, and it is unclear whether subjective assessment is accurate. We investigated the normal menstrual flow volume range in Japanese women and whether the actual measured menstrual flow volume agreed with the subjective evaluation.

Methods: We included female menstruating volunteers aged 18-49 years in this prospective observational study. Menstrual napkins were weighed before and after use for up to three cycles, and the values were recorded in an online diary.

Results: Overall, 211 participants were recruited. All items were completed by 167 participants, and 497 menstrual cycles were included in the analysis. The median total menstrual flow volume per cycle for 497 cycles was 56.7 g. The 5th-95th percentile values ranged from 15.7 to 166.4 g. The mean value was 77.6 ± 99.6 g, with no significant differences according to age group. In the 25 cycles corresponding to heavy flow, that is, above the 95th percentile of measured flow volume, 92% were underrated as "normal" by subjective evaluation, and only 8% were correctly rated as "heavy."

Conclusions: Our results clarify the actual menstrual flow volume in the Japanese population; this will contribute toward making women aware of the normal range of menstrual flow volume, which may facilitate appropriate preconception care.