Researchers have shown that the average length of the menstrual cycle increased from 15-23 years, then decreased to 45 years, and then increased again. Mean follicular phase body temperature showed no significant age-related changes, but luteal phase body temperature gradually increased up to age 29 and stabilized and began to decline after age 42.
Global statistics on the human menstrual cycle are based on outdated and limited studies from the 1950s. In a new study, researchers from Tokyo Medical and Dental University (TMDU) have leveraged the ubiquity of smartphones to collect and analyze data that shed new light on age-related and seasonal variations of menstrual rhythms and body temperature in Japanese women.
The menstrual cycle and basal body temperature are individually variable and as closely linked to physical and mental well-being as they are to social and sexual behavior. Accurate knowledge of its norms and variations is key to understanding reproductive health and defining multidimensional well-being in women. In light of this, a reassessment of current concepts is warranted.
The researchers came up with an ingenious plan to collect unprecedented amounts of data. Assistant Professor Takayuki Tatsumi, lead author, describes the scale and scope of the study. “Using a women's smartphone health app, we queried and collected menstrual parameters in terms of duration and periodicity, as well as body temperature and its variation with age, menstrual phase and seasonal indices, including ambient temperature, hours of sunshine and precipitation.”
Thanks to the benefits of anonymity and the ease of self-reporting, there was a massive response from about 310,000 Japanese women going through 6 million menstrual cycles, which required innovative Big Data analytics tools to crunch. In addition, since Japan is a largely monoethnic country stretching from subtropical to subarctic latitudes, the researchers were able to decouple the respondents' genetic backgrounds and socioeconomic factors from climatic influences.
The research team found that the average menstrual cycle gradually got longer from adolescence to age 23 (an average of 30.7 days). Thereafter, it decreased until age 45 (mean 27.3 days) and then increased. Basal body temperature remained constant (36.4 °C) during the first (follicular) phase of the cycle regardless of age, but mean body temperature in the second (luteal) phase increased gradually from adolescence to age 29 (36). .7 °C), stabilizing in the 1930s and then decreasing after the age of 42. Ambient external temperature was associated with body temperature but not cycle time.
dr. Tatsumi also explains the implications of the research results. “Contrary to historical data, our results apply specifically to contemporary Japanese women. This newer understanding of their menstrual cycles and variation in body temperature provides an authentic background for the interpretation of fertility and pregnancy research, as well as general psychosocial and sexual well-being.
