What is the menstrual cycle?
The menstrual cycle is a repeating series of changes in the ovaries and uterus that prepares the body for a possible pregnancy each month. It lasts about 28 days on average, although 21 to 35 days is normal, and it is controlled by four main hormones: FSH and LH from the pituitary gland, and oestrogen and progesterone from the ovary. The cycle begins at puberty and continues until menopause, when the ovaries stop releasing ova. Day 1 is counted from the first day of bleeding.
The four phases
- Menstruation (days 1–5): the thickened endometrium breaks down and is lost as blood if no pregnancy has occurred.
- Follicular phase (days 1–13): FSH stimulates a follicle to mature; the follicle secretes oestrogen, which rebuilds the endometrium.
- Ovulation (~day 14): a surge of LH triggers the release of a mature ovum from the ovary.
- Luteal phase (days 15–28): the empty follicle becomes the corpus luteum, which secretes progesterone to maintain the endometrium.
Hormone control
If the ovum is not fertilised, the corpus luteum breaks down, so progesterone and oestrogen fall. Without these hormones the endometrium cannot be maintained, so it is shed and the next cycle begins. If the ovum is fertilised, progesterone stays high, the endometrium is kept, and menstruation does not occur.
Example
In a 28-day cycle, ovulation is around day 14, so the fertile window is roughly the middle of the cycle. A woman with a 30-day cycle would ovulate a little later, around day 16.
Remember
FSH grows the follicle, LH triggers ovulation, oestrogen rebuilds the lining, and progesterone maintains it. A fall in progesterone starts menstruation.