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Normal Menstrual Cycle

The menstrual cycle is a rhythmic sequence of events reflecting communication between the brain, ovary and uterus.

This is called the hypothalamic-pituitary-ovarian (HPO) axis. Hormones produced in the hypothalamus and pituitary gland (brain) and follicles (ovary) coordinate the cycle events. 

Understanding the menstrual cycle unlocks a better understanding of why your doctor gives you certain medications to promote fertility. Disorders of the menstrual cycle can also contribute to infertility. 


On average, a menstrual cycle is 28 days long, but can range from 25-35 days. The cycle length is measured from the first day of bleeding in one cycle (CD1, cycle day 1) to the first day of bleeding in a subsequent cycle. There are two main phases of the menstrual cycle, the follicular phase and the luteal phase. Ovulation occurs approximately 14 days before the onset of the next menstrual period. For example, in a 28 day cycle, ovulation typically occurs around day 14; in a 26 day cycle it occurs around day 12, and in a 30 day cycle it occurs around day 16. Ovulation demarcates the transition from the follicular phase to the luteal phase.

Menstrual cycle: Two main phases