The menstrual cycle starts on the first day of menstrual bleeding, lasts 28 days on average from then.

月经周期开始于月经出血的第一天，平均持续28天。

That's why there's an increased risk of breast cancer with things that increase the number of menstrual cycles - like early age of menarche, which is the first menstrual bleeding, and late age of menopause.

这就解释了为什么月经次数较多的人患乳腺癌的几率更高，如初潮过早或停经过晚者。