Distribution of the follicular and luteal phase lengths and their age‐dependent changes in Japanese women: A large population study

Purpose The current definition of menstrual cycle length in a Japanese woman is different from those of WHO definition, and the original data are outdated. We aimed to calculate the distribution of follicular and luteal phases length in modern Japanese women with various menstrual cycles. Methods This study determined the lengths of the follicular and luteal phases of Japanese women using basal body temperature data collected via a smartphone application from 2015 to 2019, and the data were analyzed using the Sensiplan method. Over 9 million temperature readings from more than 80 000 participants were analyzed. Results The mean duration of the low‐temperature (follicular) phase averaged 17.1 days and was shorter among participants aged 40–49 years. The mean duration of the high‐temperature (luteal) phase was 11.8 days. The variance and maximum‐minimum difference of the length of the low temperature period were significant in women under 35 years old than women aged more than 35 years. Conclusions The shortening of the follicular phase in women aged 40–49 years implied a relationship with the rapid decline of ovarian reserve in these women, and the age 35 years old was turning point of ovulatory function.