Menstrual cycle length and body temperature change with age and seasons

Statistics prevalent worldwide about the human menstrual cycle are based on antiquated and restricted surveys dating from the 1950s. In a new study, researchers from Tokyo Medical and Dental University (TMDU) have harnessed the ubiquity of smartphones to amass and analyze data that throws 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 intimately linked with physical and mental wellness as it is to social and sexual behavior. Precise knowledge of its norms and variations is key to understanding reproductive health and defining multidimensional wellbeing in women. In light of this, a reappraisal of current concepts is warranted.

The researchers hit upon an ingenious scheme for gathering unprecedented volumes of data. Assistant professor Takayuki Tatsumi, lead author, describes the scale and scope of the study. "Using a smartphone healthcare app for women, we requested and collated menstruation parameters in terms of duration and periodicity as well as body temperature and its variation with age, menstrual phase and seasonal indices including environmental temperature, sunshine hours and precipitation."

Thanks to the advantages of anonymity and convenience of self-reporting, there was a massive response from about 310,000 Japanese women covering 6 million menstrual cycles, which required innovative Big Data analytical tools to crunch. Additionally, as Japan is a largely monoethnic country that spans from subtropical to subarctic latitudes, the researchers were able to disentangle the genetic backgrounds and socioeconomic factors of the respondents from climatic influences.

The research team found that the average menstrual cycle gradually lengthened from adolescence until the age of 23 years (average 30.7 days). Thereafter, it decreased until the age of 45 years (average 27.3 days), increasing subsequently. Basal body temperature remained constant (36.4°C) during the first (follicular) phase of the cycle regardless of age, but the average body temperature in the second (luteal) phase gradually increased from adolescence to the age of 29 (36.7°C), stabilizing in the thirties then declining after the age of 42. External environmental temperature was associated with body temperature but not with cycle length.

Dr. Tatsumi also explains the implications of the research findings. "Unlike historical data, our results apply specifically to contemporary Japanese women. This newer understanding of their menstrual cycle and body temperature variation provides an authentic background to the interpretation of research into fertility and pregnancy as well as overall psychosocial and sexual wellness.