Reproductive Physiology
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Direct answer
The ovarian cycle divides into a variable follicular phase, in which FSH grows a dominant follicle whose oestradiol finally crosses a threshold and flips feedback positive to trigger the LH surge about a day before ovulation, and a fixed 14-day luteal phase, in which the corpus luteum's progesterone builds a secretory endometrium and menstruation follows its failure. In the male, FSH drives Sertoli cells and spermatogenesis while LH drives Leydig testosterone. Pregnancy announces itself through hCG from the syncytiotrophoblast, which rescues the corpus luteum until the placenta assumes steroid production around 8-10 weeks.
What you must remember
- Spermatogenesis takes about 64-74 days plus epididymal transit; Sertoli cells create the blood-testis barrier and secrete inhibin B and androgen-binding protein; Leydig cells make testosterone under LH.
- The testis needs a temperature 2-3 degrees below core — hence the infertility of cryptorchidism and varicocele.
- LH surge trigger: sustained oestradiol above roughly 200 pg/mL for about 50 hours switches feedback from negative to positive; ovulation follows roughly a day after the surge begins.
- Endometrial rhythm: oestrogen builds the proliferative lining, progesterone converts it to secretory with spiral arteries, and withdrawal of both hormones menstruates it — average loss 30-40 mL, non-clotted because of fibrinolysin.
- hCG shares its alpha subunit with LH, FSH and TSH, peaks at 8-10 weeks, and is the basis of every pregnancy test; its TSH-like action explains the biochemical thyrotoxicosis of molar pregnancy.
- Luteal-placental shift at 8-10 weeks: removing the corpus luteum before this point ends the pregnancy, after it the placenta sustains progesterone alone.
- Lactation: prolactin synthesises milk, oxytocin ejects it through myoepithelial contraction on suckling; colostrum is rich in protein and secretory IgA.
- Postpartum contraception by lactational amenorrhoea is about 98 per cent effective when feeding is exclusive, amenorrhoea persists and the baby is under six months.
Working through the 28-day cycle
Day one is the first day of bleeding, by convention. Through the first week FSH recruits a cohort of follicles; the dominant one, best supplied with aromatase and FSH receptors, races ahead and suppresses the others while its oestradiol first suppresses then — past the 200 pg/mL threshold sustained for some 50 hours — stimulates LH. The mid-cycle surge loosens the cumulus, thins follicular walls and fires proteases; ovulation occurs roughly 16-24 hours after the surge's start, often with mittelschmerz and followed by a basal temperature rise of 0.3-0.5 degrees from progesterone. The ruptured follicle becomes the corpus luteum, and for exactly its 14-day lifespan it secretes progesterone that prepares a secretory endometrium; with no hCG rescue, it involutes, both steroids collapse, and the spiral arteries constrict to shed the lining — day one again.
Every clinical rule is a corollary of this timeline. Cycle-length variability comes almost entirely from the follicular phase; the luteal phase is nearly constant, which is why ovulation is dated 14 days before the next expected period, not 14 days after the last. Sperm survive three to five days in the tract while the ovum is viable barely a day. Anovulation, as in polycystic ovary syndrome — now among the commonest endocrine problems of young Indian women — collapses the cycle into chronic unopposed oestradiol with irregular, unpredictable bleeding; clomiphene or letrozole induce ovulation by lifting gonadotrophin drive. And after the menopause, average age near the late forties in Indian women, exhausted follicles remove inhibin and oestradiol so FSH climbs higher than at any other time of life — the single best laboratory marker.
Where students slip
Four slips recur. Candidates attribute the LH surge to progesterone — it is oestradiol's positive feedback that fires it, with progesterone rising only afterwards. They call the luteal phase variable, forgetting that it is the follicular phase that lengthens or shortens the cycle. They mix up hCG's roles, forgetting its shared alpha subunit explains both the pregnancy test and molar thyrotoxicosis.
Frequently asked questions
What triggers the LH surge?
Sustained oestradiol above roughly 200 pg/mL for about 50 hours converts hypothalamic-pituitary feedback from negative to positive, firing the mid-cycle surge.
Why is the luteal phase fixed at 14 days?
The corpus luteum has an intrinsic lifespan unless rescued by hCG; since it is the follicular phase that varies, cycle-length differences arise before ovulation.
What does hCG do, and when does it peak?
It rescues the corpus luteum through LH-like action, peaks at 8-10 weeks of gestation, and forms the basis of pregnancy tests; removal of the corpus luteum before the luteal-placental shift aborts the pregnancy.
Which hormones build the breast and which act during feeding?
Oestradiol grows the ducts and progesterone the alveoli; during suckling, prolactin synthesises milk while oxytocin contracts myoepithelial cells for let-down.
How reliable is lactational amenorrhoea as contraception?
About 98 per cent effective if feeding is exclusive and amenorrhoea persists within six months postpartum — a recognised Indian family-planning spacing method.
Why is FSH highest after menopause?
Exhaustion of follicles removes inhibin and oestradiol feedback, so FSH rises to lifetime-high levels — the characteristic laboratory signature.