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Embryology · General embryology: weeks 1 to 8

Ovarian cycle, ovulation and the corpus luteum !

  • Process

Each month a follicle matures, releases an egg and leaves a corpus luteum that prepares the uterus for a possible pregnancy.

Updated 2026-11-01

See it in 3D: Ovaries · Uterine tubes · Infundibulum and fimbriae · Endometrium

What it is

The ovarian cycle is driven by hypothalamic GnRH and pituitary FSH and LH. In the follicular phase several follicles grow and one becomes dominant; the LH surge triggers ovulation; in the luteal phase the corpus luteum makes progesterone that keeps the endometrium secretory. If pregnancy occurs, trophoblast hCG rescues the corpus luteum.

Key facts

  • Follicular phase: its length varies with the cycle; FSH recruits follicles and one dominates.
  • Theca interna makes androgens; granulosa converts them to estradiol (aromatase).
  • LH surge: the oocyte finishes meiosis I and ovulation follows shortly (around day 14 of a 28-day cycle).
  • Luteal phase: fairly constant, about 14 days.
  • Without fertilization the corpus luteum degenerates in about 10 days into the corpus albicans; progesterone falls and menstruation follows.
  • In pregnancy, hCG keeps the corpus luteum going until the placenta makes enough progesterone, near the end of the first trimester.

Step by step

  1. Primordial follicle: a primary oocyte surrounded by flat cells.
  2. Primary follicle: cuboidal granulosa cells and the zona pellucida appear.
  3. Secondary (antral) follicle: a fluid-filled cavity plus theca interna and externa layers.
  4. Mature (Graafian) follicle: the oocyte sits on the cumulus oophorus, ready to leave.
  5. Ovulation: the follicle ruptures and the tubal fimbriae pick up the oocyte with its corona radiata.
  6. Corpus luteum: granulosa and theca cells luteinize and secrete progesterone and estrogens.

Markers

  • GnRH
  • FSH
  • LH
  • Estradiol
  • Progesterone

Clinical notes

  • Polycystic ovary syndrome: chronic anovulation, hyperandrogenism and many small arrested follicles.
  • Mid-cycle pain (Mittelschmerz): one-sided pelvic discomfort at ovulation.
  • Corpus luteum cyst: can rupture and bleed, mimicking ectopic pregnancy.
  • Ovulation day is estimated by counting 14 days back from the next period, not forward.

Test yourself

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

Which hormone triggers ovulation?

The LH surge, which also makes the oocyte finish meiosis I.

Which phase of the ovarian cycle has a nearly constant length?

The luteal phase, about 14 days. Differences in cycle length lie in the follicular phase.

What keeps the corpus luteum alive early in pregnancy?

hCG from the syncytiotrophoblast, acting on the corpus luteum's LH receptor.

Which follicle cell converts androgens to estradiol?

The granulosa cell, through FSH-stimulated aromatase.

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← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 3: First Week of Development: Ovulation to Implantation (Wolters Kluwer, 2015); Polyendocrine metabolic ovarian syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.