Obstetrics and Gynaecology chapters
The menstrual cycle, HPO axis, puberty and menarche
The ovarian and endometrial cycles hour by hour, how the hypothalamic-pituitary-ovarian axis times ovulation, and how puberty starts and progresses to menarche.
- The ovary runs a follicular phase (variable length), ovulation about 24-36 hours after the LH surge, and a luteal phase fixed at about 14 days; so ovulation day = cycle length minus 14.
- The endometrium answers with proliferative (oestrogen) then secretory (progesterone) changes; progesterone withdrawal when the corpus luteum dies causes menstruation.
- Puberty starts with night-time GnRH pulses; the first sign in girls is breast budding (thelarche), and menarche comes about 2-3 years later (mean about 12.8 years), with early cycles often anovulatory.
On this page
Overview
A menstrual cycle is counted from the first day of bleeding to the day before the next bleed. The average is about 28 days. Two linked cycles run at the same time:
- The ovarian cycle: follicular phase → ovulation → luteal phase.
- The endometrial cycle: menstrual → proliferative → secretory phase.
Both are driven by the hypothalamic-pituitary-ovarian (HPO) axis (see the chapter on reproductive hormones). PGET questions test the timing of events, which hormone does what, and the order of pubertal changes.
The ovarian cycle
Follicular phase (day 1 to ovulation)
- Early follicle growth does not need gonadotrophins, but without FSH and LH follicles stop at the preantral stage and die (atresia).
- As the previous corpus luteum fails, oestradiol, progesterone and inhibin are low, so FSH rises in the first days of the cycle. A group of small antral follicles starts to grow.
- LH makes theca cells produce androgens; FSH makes granulosa cells aromatise them to oestradiol.
- Rising oestradiol and inhibin lower FSH. Only the follicle with the best aromatase activity and most FSH-induced LH receptors survives the falling FSH: the dominant follicle. The rest undergo atresia.
- This phase varies in length between women and between cycles; it is the part that makes a cycle long or short.
Ovulation
- The dominant follicle reaches about 20 mm.
- Its high oestradiol, sustained for about 2 days, switches to positive feedback: the LH surge.
- Ovulation follows about 24-36 hours after the surge begins. Urine ovulation kits detect this LH rise.
- The LH surge:
- lets the oocyte resume meiosis (it has been arrested in prophase I since fetal life; it completes meiosis I and is released as a secondary oocyte);
- luteinises granulosa cells, so they start making progesterone;
- with FSH, prostaglandins and proteolytic enzymes, breaks down the follicle wall.
Prostaglandins are needed for follicle rupture. NSAIDs such as ibuprofen or aspirin taken around mid-cycle can stop the follicle releasing its egg, so women trying to conceive should avoid them at that time.
- Some women feel mid-cycle pain at ovulation (mittelschmerz). A small rise in androgens near ovulation may raise libido at the most fertile time.
Luteal phase (ovulation to next period)
- The ruptured follicle becomes the corpus luteum (yellow from lipid and pigment), richly vascularised under VEGF.
- It secretes progesterone (peak about 7 days after ovulation), oestradiol and inhibin A. These suppress FSH and LH so no new follicle matures.
- The luteal phase lasts about 14 days in most women and changes little.
- No pregnancy: the corpus luteum regresses (luteolysis) to the corpus albicans; hormone levels fall; FSH rises again and a new cycle starts.
- Pregnancy: hCG from the implanting embryo acts on LH receptors and rescues the corpus luteum until the placenta takes over at about 8 weeks.
"The luteal phase is a 14-day timer." The follicular phase is the flexible part.
Worked examples
Ovulation day = cycle length − 14
28-day cycle: 28 − 14 = day 14
35-day cycle: 35 − 14 = day 21
24-day cycle: 24 − 14 = day 10
Take it about 7 days before the next expected period
28-day cycle: 28 − 7 = day 21
32-day cycle: 32 − 7 = day 25
"Day 21 progesterone" is only day 21 in a 28-day cycle. In a long cycle, testing on day 21 is too early and gives a falsely low result.
Blocking prostaglandins can leave a mature follicle unruptured even after a normal LH surge, so progesterone may rise although no egg is released.
Fertile window
- The egg can be fertilised for only about 12-24 hours after release.
- Sperm survive in the female tract for up to about 72 hours (some sources say longer in good cervical mucus).
- So the fertile days are the few days before ovulation plus the day of ovulation. For a 28-day cycle this is about days 12-15.
The endometrial cycle
Structure
- Functionalis: the upper layer that changes each month and is shed. It appears after puberty and is lost after the menopause.
- Basalis: the deep layer next to the myometrium. It is stable, holds the stem cells, and regrows the functionalis after each period.
Destroying the basalis (over-vigorous curettage, infection) leaves scarring and Asherman syndrome. Endometrial ablation works on the same principle on purpose.
Phases
| Endometrial phase | Ovarian phase | Hormone | Changes |
|---|---|---|---|
| Menstrual (days 1-5) | Early follicular | Falling progesterone and oestrogen | Functionalis shed over 1-3 days; bleeding stops by about day 5-6 as it regrows |
| Proliferative | Follicular | Oestrogen | Glands and stroma grow; epithelium becomes pseudostratified with mitoses; thickness rises from about 0.2 mm to 3.5-5 mm; glands lengthen |
| Secretory | Luteal | Progesterone | Growth stops; glands become tortuous and secrete; spiral arteries coil; stroma becomes decidua; pinopodes on days 19-21 |
- The short time when the endometrium can accept an embryo is the window of implantation (mid-luteal).
- An endometrial biopsy taken late in the cycle that still shows proliferative glands means the cycle was anovulatory.
Menstruation
- The corpus luteum dies, so progesterone and oestrogen fall.
- Spiral arterioles constrict → ischaemia of the functionalis.
- Inflammatory cells, prostaglandins and matrix-breaking enzymes break down the tissue.
- The functionalis is shed.
- Bleeding stops through platelet plugs, fibrin and thrombus, myometrial contraction, and regrowth of the surface.
- Normal blood loss is about 40 mL. Menstrual blood normally does not clot because the endometrium releases fibrinolysins; passing clots suggests heavy loss.
- Prostaglandins also cause period cramps; this is why NSAIDs treat both painful and heavy periods.
- The pill and HRT copy this: stopping the hormones for a week gives a withdrawal bleed.
Cervical, vaginal and temperature changes
| Marker | Before ovulation (oestrogen) | After ovulation (progesterone) |
|---|---|---|
| Cervical mucus | Clear, profuse, stretchy; sperm can pass | Thick, sticky, scanty; hostile to sperm |
| Basal body temperature | Lower | Rises by about 0.3 °C (0.5 °F) and stays up until the period |
| Cervix | Soft, higher, os more open | Firmer, lower, os closed |
Basal temperature that stays raised for more than about 2 weeks after ovulation suggests pregnancy (the corpus luteum keeps making progesterone).
Puberty
Mechanism
- In childhood the HPO axis is quiet: GnRH, FSH and LH are very low.
- From about 8-9 years, GnRH pulses grow in size and frequency, first during sleep, later through the day.
- FSH and LH then rise, follicles grow, and ovarian oestrogen drives the body changes.
- The trigger is a maturation of brain centres. Kisspeptin signalling and leptin (body fat) are key; genetics, race and body weight change the timing.
- Growth hormone rises too, giving the growth spurt.
Sequence in girls
| Order | Event | Driven by |
|---|---|---|
| 1 | Thelarche: breast bud (Tanner 2 breast) | Ovarian oestrogen |
| 2 | Pubarche/adrenarche: pubic and axillary hair | Adrenal androgens |
| 3 | Growth spurt (peak height velocity) | Oestrogen and growth hormone |
| 4 | Menarche | Oestrogen-primed endometrium shedding |
- Breast budding comes about 2-3 years before menarche.
- Mean age of menarche is about 12.8 years (Guyton gives a range of 11-16).
- It can take over 3 years for cycles to become regular. Early cycles are often anovulatory: the positive feedback LH surge is not yet mature, so bleeding is irregular and sometimes heavy, and usually painless (period pain often starts when ovulatory cycles begin).
- Oestrogen also changes the uterus from a small tube to a pear shape with the body about twice the length of the cervix, thickens the vaginal epithelium, and finally fuses the epiphyses.
"Breasts, Hair, Height, Period" in that order.
Pubic hair is mainly an adrenal androgen effect, not an ovarian oestrogen effect. A girl with complete androgen insensitivity therefore has breasts but very little pubic hair.
Tanner staging (girls)
| Stage | Breast | Pubic hair |
|---|---|---|
| 1 | Prepubertal; no glandular tissue | None |
| 2 | Breast bud under the areola (first sign) | Sparse, fine, along the labia |
| 3 | Breast tissue beyond the areola | Darker, coarser, spreading over the mons |
| 4 | Areola and papilla form a secondary mound | Adult-type hair, but limited area |
| 5 | Adult contour; areola back in line with breast | Adult triangle spreading to the thighs |
Abnormal timing (precocious and delayed puberty) is covered in the chapter on disorders of sex development.
Premenstrual syndrome (PMS)
- Physical and mood symptoms in the luteal phase that settle once bleeding starts: low mood, irritability, anxiety, bloating, breast tenderness, food cravings, headache, poor sleep.
- Cause: cyclical ovarian activity acting on serotonin and GABA pathways in sensitive women.
- Diagnosis needs a prospective symptom diary over at least two cycles.
- Management in steps:
- First line: vitamin B6, regular exercise, CBT, a newer combined pill, low-dose SSRI (continuous or luteal only).
- Second line: transdermal oestradiol with progestogen protection, higher-dose SSRI.
- Third line: GnRH analogue with add-back HRT.
- Last resort: hysterectomy with removal of both ovaries, after a GnRH test of benefit.
Expect: the luteal phase as a fixed 14 days and ovulation day calculated as cycle length minus 14; timing of the mid-luteal progesterone test; the LH surge 24-36 hours before ovulation and its effects (meiosis resumes, luteinisation); follicle size about 20 mm; atresia of non-dominant follicles; corpus luteum rescue by hCG; progesterone withdrawal and spiral artery spasm as the cause of menstruation; functionalis versus basalis; prostaglandins in ovulation and period pain; puberty order (thelarche first, adrenal androgens for pubic hair, menarche after the growth spurt) and anovulatory early cycles.
Quick revision
- Average cycle 28 days; luteal phase about 14 days; follicular phase varies.
- Ovulation day = cycle length − 14; mid-luteal progesterone = 7 days before next period.
- LH surge → ovulation in 24-36 hours; follicle about 20 mm.
- Oocyte arrested in prophase I until the LH surge.
- Proliferative = oestrogen; secretory = progesterone.
- Menstruation: progesterone falls → spiral arteriole spasm → ischaemia → shedding.
- Puberty: GnRH pulses (sleep first) → thelarche → pubarche → growth spurt → menarche.
- Mean menarche about 12.8 years; early cycles anovulatory.
Written by The House Job from standard references (Gynaecology by Ten Teachers; Guyton and Hall Textbook of Medical Physiology).
FAQ
Why does cycle length vary while the luteal phase stays about 14 days?
The corpus luteum has a built-in lifespan of about two weeks unless hCG rescues it; differences in cycle length come from how long the follicle takes to mature.
Why does basal body temperature rise after ovulation?
Progesterone from the corpus luteum acts on the hypothalamic temperature centre and raises temperature by a fraction of a degree until it falls before the period.
Why are the first periods after menarche often irregular and painless?
The positive feedback needed for an LH surge is still immature, so many early cycles do not ovulate; without ovulation there is little luteal prostaglandin build-up and less cramping.
What is the window of implantation?
A short mid-luteal period when progesterone has made the endometrium receptive (pinopodes, secretory glands, decidual change), during which an embryo can attach and invade.