Distribution of early age onset of ovulatory cycles by year of birth. Early age onset of ovulatory cycles means the woman who experienced younger age at menarche at ten years old or younger and started a regular cycle within one year. Cochran-Armitage Trend Test Z = -7.40, p < .0001.
Hosokawa et al. BMC Women's Health 2012 12:19 doi:10.1186/1472-6874-12-19