Menstrual cycle (HPG axis)
The hormone loop that runs the 28-day cycle.
One ovulation and a receptive endometrium per cycle
This mnemonic, unpacked word by word →
The same loop drives two behaviours. For most of the cycle estrogen and progesterone feed back negatively on the hypothalamus and pituitary. For about two days before day 14, sustained high estrogen flips that feedback positive, LH surges, and the follicle ruptures. The leftover follicle becomes the corpus luteum, and its progesterone runs the second half of the cycle. Every hormonal contraceptive works by keeping the feedback negative so the surge never comes.
GnRH only works in pulses. A constant level desensitises the pituitary and switches FSH and LH off; pulses are what switch them on.
The two-cell model: theca cells make androgens under LH, and granulosa cells aromatise those androgens to estrogen under FSH. Neither cell type can do it alone.
Estrogen inhibits LH release all cycle, until it stays high for about two days near day 12. That sustained high level flips the pituitary from inhibition to excitation, and LH spikes.
The surge begins about a day before the oocyte is released around day 14. Ovulation-predictor kits detect LH (students often expect FSH).
The ruptured follicle reorganises into the corpus luteum, which secretes progesterone and some estrogen. Without hCG from an implanted embryo it degenerates after about 14 days, progesterone collapses, and the lining sheds. The luteal phase is the fixed half: cycle length varies in the follicular phase.