Background: Early life-stage exposure to estrogen increases the risk of breast cancer. The objective of this study was to investigate the age at menarche and time to onset of regular menstrual cycles for Japanese women born between 1930 and 1985.
Methods: A cross-sectional study was designed using data from the baseline survey of the Japan Nurses' Health Study. The data from 48,104 female nurses were analyzed. To view trends in age at menarche, the distribution of age at menarche was calculated for each birth year cohort. The distribution of time to onset of regular menstrual cycles was calculated for each birth year cohort. To estimate whether high-risk group of the estrogenic dependent disorders increase with succeeding generations, we defined the women who experienced menarche at ten years old or younger and started a regular cycle within one year as early age onset of ovulatory cycles.
Results: Average ages at menarche were as follows: 13.8 years for those born in the 1930s (n = 113), 13.3 years for the 1940s (n = 4,751), 12.8 years for the 1950s (n = 15,844), 12.3 years for the 1960s (n = 20,547), 12.2 years for the 1970s (n = 6,568), and 12.2 years for the 1980s (n = 281). The proportion of women who experienced the onset of regular menstrual cycles 1 year after menarche was 29.3% for those born in the 1930s, but decreased to 11.9% for the 1980s. On the other hand, the proportion of women who did not have regular menstrual cycles was 10.4% for those born in the 1930s, but rose to 19.8% in 1980s. The proportion of women who experienced menarche at 10 years old and started regular menstrual cycles within one year increased over time: the percentage was 0.0%, 0.4%, 0.6%, 1.1%, 1.3%, and 2.1% for the women born in 1930s, 1940s, 1950s, 1960s, 1970s, and 1980s, respectively.
Conclusions: The age at menarche of Japanese women born between 1930 and 1985 decreased, but the onset of regular menstrual cycling is delayed; so that the distribution of the start time of ovulatory cycles may have spread for younger generations. Those suggest that the high-risk group of estrogenic dependent diseases among Japanese women may increase in the near future.