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PMS tracks a progesterone crash
The first half
The cycle starts on day zero, when estrogen and progesterone are so low they can't be measured. Rising follicle stimulating hormone pushes estrogen up through the first 14 days, the follicular phase, and around day 14 an ovary releases a follicle.
The second half
Luteinizing hormone then prepares the lining of the uterus for a possible pregnancy. Progesterone, idle for two weeks, begins to rise and dominates the second half, the luteal phase. By about day 21 the body has figured out whether she is pregnant, and usually she is not, so progesterone and estrogen crash and the lining is shed.
The PMS window
The last seven days of the cycle are when PMS symptoms appear in a susceptible woman, and the speed of the progesterone drop is probably affecting something in her brain. Holding progesterone steady through those days makes the symptoms go away, which indicates a real physiological effect.
How does progesterone affect mood?
The body converts progesterone into allopregnanolone, which boosts the activity of GABA-A receptors, the brain's main inhibitory system. A fall in allopregnanolone reduces that calming signal. In a 1998 New England Journal of Medicine study, Peter Schmidt and colleagues suppressed the ovarian cycle in women with and without premenstrual syndrome, then added back hormones. Symptoms returned only in the women with PMS.
Why is the second half of the cycle steady?
The luteal phase lasts about 14 days in most women because the corpus luteum, the structure left behind by the released follicle, has a fixed lifespan of roughly that long. Variation in total cycle length comes mostly from the first half. If an embryo implants, it secretes human chorionic gonadotropin, which keeps the corpus luteum alive. That hormone is what home pregnancy tests detect.
Why does estrogen trigger ovulation?
Estrogen usually suppresses luteinizing hormone, but once it stays high for a couple of days late in the follicular phase, its effect on the brain flips to positive feedback. The result is a surge in luteinizing hormone that peaks around 24 to 36 hours before the follicle ruptures. This reversal is one of the few positive feedback loops in human hormone control.
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