Physiology · Reproduction, pregnancy and the fetus

The menstrual cycle: ovary and endometrium !

Regulation
Exam priority: high

About once a month the ovary ripens an egg and the uterine lining thickens; if no pregnancy occurs, hormones fall and the lining sheds.

Updated 2026-11-01

Also called: Ovarian cycle, Endometrial cycle

See it in 3D: Ovaries · Endometrium · Body of uterus · Pituitary gland · Hypothalamus

What it is

The menstrual cycle couples two clocks. In the ovary, a follicle grows (follicular phase), releases its oocyte (ovulation) and becomes the corpus luteum (luteal phase). In the uterus, estradiol builds the endometrium (proliferative phase) and progesterone makes it receptive (secretory phase). If no hCG rescues the corpus luteum, progesterone falls and menstruation starts.

How it works, step by step

Late in the previous cycle progesterone and inhibin A fall, so FSH rises and recruits a cohort of antral follicles.
FSH induces aromatase in granulosa cells, which convert androgens from LH-stimulated theca cells into estradiol.
The follicle that makes the most estradiol and gains LH receptors becomes dominant; falling FSH lets the others undergo atresia.
Rising estradiol makes the endometrium proliferate and the cervical mucus thin and stretchy.
Sustained high estradiol flips pituitary feedback to positive: a large LH surge (with a smaller FSH peak) follows.
The LH surge restarts meiosis in the oocyte, weakens the follicle wall and causes ovulation about 34–36 h later.
The ruptured follicle luteinizes into the corpus luteum, which secretes progesterone, estradiol and inhibin A.
Progesterone converts the endometrium to a secretory, glycogen-rich layer ready for implantation.
Without hCG, the corpus luteum regresses after about 12–14 days; progesterone withdrawal triggers shedding of the functional layer.

Hormone axes and feedback

Hypothalamic–pituitary–ovarian axisLesion None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

Stimulates (+)
Inhibits / negative feedback (−)
Positive feedback (high E₂ at mid-cycle)
Level ↑
Level ↓

Tap the ovary and see the dashed arrow: high estradiol switches to positive feedback and triggers the LH surge.

Reference values (adult, FIGO ranges)

Parameter	Value	Note
Cycle length	≈24–38 days	normal range; median ≈28 days
Duration of menses	≤8 days	usually 4–6 days
Menstrual blood loss	≈30–40 mL	>80 mL is objectively heavy
Luteal phase	≈12–14 days	fairly constant
Variation in cycle length	≤7–9 days	between shortest and longest cycle

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Regulation

+FSH: Recruits follicles, induces aromatase and inhibin in granulosa cells.
+High sustained estradiol (mid-cycle): Positive feedback: LH surge and ovulation.
−Low or moderate estradiol (early follicular): Negative feedback on FSH and LH.
−Progesterone with estradiol (luteal phase): Slows GnRH pulses and blocks a new surge.
−Inhibin B, then inhibin A: Lower FSH and help select one dominant follicle.
+hCG from the embryo: Rescues the corpus luteum and keeps progesterone high in early pregnancy.

Phases side by side (28-day cycle)

Days	Ovary	Endometrium	Dominant hormone
1–5	Early follicular: cohort recruited	Menstrual: functional layer sheds	FSH rising; estradiol and progesterone low
6–13	Late follicular: dominant follicle	Proliferative: glands and stroma grow	Estradiol rising
≈14	Ovulation	Late proliferative	LH surge
15–28	Luteal: corpus luteum	Secretory: glycogen-rich glands, spiral arteries	Progesterone (plus estradiol)

Clinical notes

Polycystic ovary syndrome: high LH:FSH, ovarian androgens and anovulation give irregular cycles, hirsutism and unopposed estrogen on the endometrium.
Primary dysmenorrhea is driven by endometrial prostaglandins; that is why prostaglandin-synthesis inhibitors relieve it.
Abnormal uterine bleeding is classified by cause (structural vs non-structural, PALM-COEIN); ovulatory dysfunction is a common non-structural cause.

Test yourself

Think of the answer, then tap the question to see it.

A woman has cycles of 35 days. Roughly on which day does she ovulate, and why?

Around day 21: the luteal phase is fairly fixed at about 14 days, so ovulation is about 14 days before the next period. The longer follicular phase absorbs the extra days.

Estradiol usually inhibits LH. What makes it trigger the mid-cycle LH surge instead?

A high level sustained for about 36–48 hours switches the hypothalamus and pituitary to positive feedback, so GnRH and LH rise sharply.

What directly triggers menstruation at the end of a non-fertile cycle?

Regression of the corpus luteum: progesterone and estradiol fall, spiral arteries constrict, the functional layer becomes ischemic and sheds, helped by prostaglandins.

Why does a rise in basal body temperature suggest ovulation has already occurred?

The rise is caused by progesterone from the corpus luteum, which only forms after ovulation; it confirms ovulation in retrospect but does not predict it.

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The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 27.2 Anatomy and Physiology of the Ovarian Reproductive System (OpenStax (Rice University), 2022); Abnormal Uterine Bleeding (StatPearls (NCBI Bookshelf), 2025); Embryology, Ovarian Follicle Development (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 82). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.