Reproduction

From puberty (around age 11–14) until menopause (around age 50), the female body runs a roughly 28-day cycle that prepares for possible pregnancy each month. If pregnancy does not happen, the body sheds the prepared lining and starts again.

What happens during the cycle

The cycle is usually counted from day 1, which is the first day of menstruation (the period).

• Days 1–5: Menstruation. The thick uterine lining that was prepared during the previous cycle breaks down and is shed through the vagina. About 30–80 ml of blood and tissue are lost.
• Days 5–14: Lining rebuilds, egg matures. The uterus rebuilds a thick blood-rich lining ready for a possible fertilised egg. Meanwhile, one egg matures in one ovary.
• Day 14: Ovulation. The mature egg is released from the ovary into the oviduct. This is the only fertile day in the cycle.
• Days 14–28: Lining maintained, waiting. The lining stays thick, waiting for a fertilised egg to implant.
• Day 28: If no fertilisation has happened, the lining breaks down and the next cycle begins. If fertilisation has happened, the lining stays intact and pregnancy continues.

The four hormones that run the cycle

Four hormones coordinate the menstrual cycle, working in a carefully timed sequence. Two come from the pituitary gland in the brain, and two come from the ovaries.

The interaction between the four hormones

1. At the start of the cycle, the pituitary releases FSH.
2. FSH causes a follicle in the ovary to develop, and the follicle starts releasing oestrogen.
3. Rising oestrogen levels:
   • Rebuild the uterine lining
   • Inhibit FSH (so no further follicles develop)
   • Stimulate LH release as oestrogen peaks
4. The LH surge around day 13–14 triggers ovulation.
5. The empty follicle becomes the corpus luteum, which secretes progesterone.
6. Progesterone maintains the thick uterine lining and inhibits both FSH and LH.
7. If the egg is not fertilised, the corpus luteum breaks down, progesterone levels fall, and the uterine lining breaks down → menstruation begins → the cycle starts again.