Ovarian cycle
Also known as: follicular phase, luteal phase, ovulation, LH surge
The ≈28-day sequence of follicle growth (follicular phase), ovulation and corpus luteum function (luteal phase) driven by the hypothalamic–pituitary–ovarian axis, which in turn drives the uterine cycle.
Overview
Luteal–follicular transition: regression of the previous corpus luteum lowers estradiol, progesterone and inhibin A; GnRH pulse frequency increases and FSH rises, recruiting a cohort of antral follicles. Early–mid follicular phase (day 1 to ≈day 7): follicles grow and secrete increasing estradiol and inhibin B; a dominant follicle is selected (≈days 5–7), while falling FSH starves the rest into atresia. Late follicular phase: the dominant follicle's estradiol rises steeply; once it exceeds ≈200 pg/mL for ≈36–48 h, feedback becomes positive, triggering the LH surge (with a smaller FSH peak) on ≈day 13. Ovulation follows ≈24–36 h after the surge begins (≈10–12 h after the LH peak).
Luteal phase (≈14 days, the most constant part of the cycle): the corpus luteum secretes progesterone (peak ≈7 days after ovulation, day ≈21), estradiol and inhibin A; their combined negative feedback keeps FSH and LH low, and progesterone slows GnRH pulses. Without hCG the corpus luteum regresses, hormone levels fall and the next cycle begins.
The follicular phase varies in length (≈10–16 days or more), so ovulation occurs about 14 days before the next period rather than on a fixed cycle day. Basal body temperature rises ≈0.3–0.5 °C after ovulation (progesterone), and cervical mucus becomes thick.
- Location
Ovaries, under the control of the hypothalamus and anterior pituitary.
Parts & subdivisions
- FSH-driven follicle recruitment, dominance and estradiol rise; variable length.
- Triggered by the LH surge; release of the secondary oocyte.
- Corpus luteum secretes progesterone; ≈14 days.
Function
- Matures and releases one oocyte per cycle.
- Generates the cyclic estrogen and progesterone pattern that prepares the uterus for implantation.
- Physiology
Hormone curves: FSH — early follicular rise, mid-follicular fall, small mid-cycle peak, low luteal, late-luteal rise. LH — low until a sharp ≈10-fold mid-cycle surge lasting ≈48 h. Estradiol — rises through the late follicular phase to peak ≈1 day before the LH surge, dips after ovulation, second plateau mid-luteal. Progesterone — low in the follicular phase, starts rising just before ovulation, peaks mid-luteal (>≈30 nmol/L confirms ovulation), falls before menses. Inhibin B peaks in the follicular phase; inhibin A in the luteal phase.
Clinical significance
Anovulation
Commonest causes: PCOS, hypothalamic amenorrhoea (low weight, exercise, stress), hyperprolactinaemia, thyroid disease and premature ovarian insufficiency. Anovulatory cycles are irregular and may cause heavy bleeding from unopposed estrogen.
Fertility awareness
Ovulation can be predicted by urinary LH kits, mid-cycle changes in cervical mucus (clear, stretchy) and the post-ovulatory temperature rise; the fertile window is the 5 days before ovulation plus the day of ovulation.
Test yourself
At what stage is a human oocyte arrested from fetal life until the LH surge?
- Metaphase II
- Prophase I (diplotene)
- Anaphase I
- G1 of the mitotic cycle
Answer: Prophase I (diplotene)
Primary oocytes arrest in the diplotene (dictyate) stage of prophase I. The LH surge triggers completion of meiosis I; the secondary oocyte then arrests in metaphase II until fertilisation.
What triggers the mid-cycle LH surge?
- Falling progesterone
- A sustained high estradiol level switching feedback from negative to positive
- A fall in inhibin A
- Rising prolactin
Answer: A sustained high estradiol level switching feedback from negative to positive
When estradiol from the dominant follicle stays above ≈200 pg/mL for ≈36–48 h, it acts positively on the hypothalamus and pituitary, producing the LH surge that leads to ovulation ≈24–36 h later.
A woman has oligomenorrhoea and hirsutism; ultrasound shows 25 follicles in each ovary. Which statement about PCOS is correct?
- Diagnosis requires all three Rotterdam criteria
- Chronic anovulation exposes the endometrium to unopposed estrogen, increasing endometrial cancer risk
- FSH is typically much higher than LH
- Insulin sensitivity is usually increased
Answer: Chronic anovulation exposes the endometrium to unopposed estrogen, increasing endometrial cancer risk
PCOS needs two of three criteria (anovulation, hyperandrogenism, polycystic morphology). Without ovulation there is no progesterone to oppose estrogen, so endometrial hyperplasia and carcinoma risk rises. LH is often high relative to FSH, and insulin resistance is typical.
Compiled from standard anatomy and physiology references — see the Reproductive System sources. For education only; not medical advice.