Menstrual Cycle Phases
The menstrual cycle runs through a variable-length follicular phase, an LH-triggered ovulation, and a luteal phase of roughly fourteen days dominated by progesterone from the corpus luteum.
The cycle has two hormonally distinct halves separated by ovulation. Through the follicular phase FSH recruits antral follicles, a dominant follicle emerges and its granulosa cells raise estradiol while progesterone stays low. Once estradiol is high enough for long enough, pituitary feedback flips from negative to positive and an LH surge ruptures the follicle. The remnant becomes the corpus luteum and secretes progesterone until it regresses, withdrawing support from the endometrium and producing menses.
Length is asymmetric, and that matters for study design. The luteal phase is close to fixed at about fourteen days; almost all the variation in a 21-to-35-day cycle lives in the follicular phase, so counting backwards from the next period locates ovulation more reliably than counting forwards from the last. Ovulation follows the LH surge by roughly a day and a half. A mid-luteal progesterone, drawn about seven days after presumed ovulation, is the standard confirmation that ovulation happened at all.
Phase therefore determines what a blood test means. Estradiol near the lower limit is normal in the early follicular phase and abnormal at mid-cycle; progesterone is uninterpretable outside the luteal window; SHBG, thyroid binding proteins and inflammatory markers all shift across the cycle. Trials in premenopausal women either standardise the sampling day or accept noise wide enough to bury a modest treatment effect.
The common misuse is comparing hormone results taken on unmatched cycle days and calling the difference a treatment response; grey-market hormone panels rarely record the cycle day at all. A second error is assuming a regular cycle proves ovulation. Anovulatory cycles can bleed on schedule, which is why the mid-luteal progesterone rather than the calendar is the evidence.