Research reveals that average menstrual cycle length increases from ages 15-23, shortens until age 45, then lengthens again. Follicular phase basal body temperature shows no significant age-related changes, while luteal phase temperature rises gradually until age 29, stabilizes, and declines after age 42.
Global data on menstrual cycles stems from limited 1950s studies. Researchers at Tokyo Medical and Dental University (TMDU) used smartphone health apps to gather fresh insights into age-related and seasonal variations in menstrual rhythms and body temperature among Japanese women.
The menstrual cycle and basal body temperature vary individually and are closely tied to physical, mental, social, and sexual well-being. Understanding their norms and fluctuations is essential for reproductive health and holistic women's wellness, calling for updated research.
Led by Assistant Professor Takayuki Tatsumi, the team collected vast data via a women's health app. "We gathered menstrual duration, periodicity, body temperature variations by age, phase, and seasonal factors like ambient temperature, sunshine hours, and precipitation," Tatsumi explains.
Anonymity and ease of reporting drew data from about 310,000 Japanese women across 6 million cycles, analyzed with advanced Big Data tools. Japan's largely monoethnic population and diverse climates allowed separation of genetic, socioeconomic, and environmental influences.
Average cycle length lengthened from adolescence to age 23 (30.7 days), shortened to age 45 (27.3 days), then increased. Follicular phase basal body temperature stayed steady at 36.4°C across ages, but luteal phase rose to 36.7°C by age 29, held steady in the 30s, and fell after 42. External temperature influenced body temperature but not cycle length.
"Unlike outdated data, these findings reflect modern Japanese women," says Dr. Tatsumi. "They offer a reliable foundation for fertility, pregnancy studies, and psychosocial-sexual well-being."
