Menstrual Cycle and Hormones

The menstrual cycle — Core

The menstrual cycle is a repeating cycle of changes in the ovary and the lining of the uterus, and it exists to prepare the body for the possibility of pregnancy each cycle. A simplified 28-day cycle is a useful teaching model, but it’s worth remembering it’s a model — actual cycle length varies between individuals and between cycles.

Following the stages in order: menstruation is the shedding of the uterine lining, conventionally counted as beginning on day one, occurring if pregnancy hasn’t taken place. While that’s happening, an egg begins maturing inside a follicle in an ovary. The uterine lining then repairs and thickens again, rebuilding well before it’s actually needed. Around the middle of a simplified cycle, ovulation releases a mature egg from an ovary. If pregnancy doesn’t occur, the uterine lining — which had remained thickened after ovulation in case implantation happened — eventually breaks down again, and a new cycle begins.

Notice that the lining rebuilds beforeovulation, not after — it’s prepared in advance in case a fertilised egg needs somewhere to implant, not assembled in a rush once that need actually arises.

Hormonal control of the cycle — Supplement

Four hormones control the cycle by acting on each other in sequence, and the relationships between them matter far more than memorising four isolated definitions. FSH (follicle-stimulating hormone), produced by the pituitary gland, stimulates a follicle and its egg to develop in the ovary, and stimulates that ovary to produce oestrogen. As oestrogen — produced mainly by the developing follicle — rises, it does two things: it stimulates the uterine lining to repair and thicken, and it inhibits further FSH release. Once oestrogen reaches a high enough level, it triggers a surge in LH (luteinising hormone), also produced by the pituitary gland. That LH surge is what directly triggers ovulation, and it also drives the formation of the corpus luteum from the remains of the follicle. The corpus luteum then produces progesterone, which maintains the thickened uterine lining and inhibits both FSH and LH, preventing another follicle from developing too soon.

The exam-critical chain, in order: FSH triggers follicle development and oestrogen production; oestrogen thickens the uterine lining and, at a high enough level, triggers the LH surge; the LH surge directly triggers ovulation and forms the corpus luteum; the corpus luteum produces progesterone, which maintains the lining. Collapsing this into “oestrogen causes ovulation” skips the LH surge entirely, and that step is exactly what Supplement-level questions are usually checking for.

If pregnancy doesn’t occur, the corpus luteum degenerates, progesterone and oestrogen levels fall, the uterine lining is no longer maintained, menstruation follows, and FSH is free to rise again as a new cycle begins.

Hormones during pregnancy — Supplement

If pregnancy does occur, the uterine lining needs to stay intact rather than break down, and the hormone system adjusts accordingly. Progesterone continues to maintain the uterine lining, and as pregnancy progresses the placenta becomes an increasingly important source of both oestrogen and progesterone, alongside the ovary. The sustained high levels of these hormones during pregnancy suppress FSH and LH, which is what prevents a new follicle developing and a new ovulation cycle beginning while pregnancy is already underway.

Where this fits

This hormone system is what makes fertilisation, implantation and pregnancy biologically possible in the first place — the uterine lining these hormones build is exactly what an embryo implants into.