What the cycle is
The cyclic process that takes place in association with the reproductive system of females who are sexually mature — that is, who have reached puberty — is called the menstrual cycle.
One menstrual cycle takes approximately 28 days to complete. The pair of ovaries that release the ova, which are the female reproductive cells, become active alternately — one ovary in one cycle, the other in the next.
Two places, one cycle
The series of events in the menstrual cycle takes place in association with two main sites:
- changes that occur within the ovary;
- changes that occur within the uterus.
They are described separately on the next two pages, but they are not two cycles. They run at the same time, on the same 28 days, driven by the same hormones — the ovary prepares an ovum, and the uterus at the same time prepares a place for it to settle.
Approximately 28 days
The number is an average, not a rule. Cycle length varies between people and between one cycle and the next in the same person. The day numbers given on the following pages are typical figures for a 28-day cycle.