The menstrual cycle is the monthly hormonal process that prepares the body for potential pregnancy. The average cycle is 28 days, but normal cycles range from 21 to 35 days.
The four phases
- Menstruation (days 1–5): The uterine lining sheds if pregnancy has not occurred. Day 1 is the first day of the period.
- Follicular phase (days 1–13): Oestrogen rises, stimulating follicle development in the ovaries.
- Ovulation (day 14 on average): A mature egg is released from the ovary. The fertile window is approximately 5 days before and 1 day after ovulation.
- Luteal phase (days 15–28): Progesterone rises to prepare the uterine lining. If no fertilisation occurs, both hormones drop and menstruation begins.
Factors affecting cycle length
Stress, significant weight changes, illness, exercise levels and hormonal conditions (PCOS, endometriosis, perimenopause) can all affect cycle length and regularity. A cycle that varies by more than 7 days or produces very heavy bleeding should be discussed with a GP.