Menstrual cycle
Menstrual cycle: ovarian and endometrial phases; ovulation
Each cycle the ovary selects one follicle, ovulates it after the LH surge and turns it into a corpus luteum that lives about 14 days; the endometrium proliferates under oestrogen, becomes secretory under progesterone and is shed when progesterone falls. CPSP asks about the timing of ovulation, the fixed luteal phase, the signs of ovulation and the mechanism of menstruation.
Timing
| Phase | Length | Controlled by |
|---|---|---|
| Follicular (menstrual and proliferative) | Variable; explains differences in cycle length | FSH, then oestradiol |
| Luteal (secretory) | Fixed, about 14 days | Progesterone from the corpus luteum |
- Ovulation day = cycle length minus 14: day 14 in a 28-day cycle, day 10 in 24 days, day 21 in 35 days.
- Progesterone to confirm ovulation: 7 days before the expected period (day 21 of 28, day 25 of 32), when it peaks about 7 days after ovulation.
- A positive pregnancy test means the woman has stayed in the luteal phase: hCG has rescued the corpus luteum.
Follicles
- Oocyte numbers: about 7 million at 20 weeks of fetal life, 1–2 million at birth, about 400,000 at puberty; only about 400 ovulate; the rest undergo atresia.
- Stages: primordial, primary, secondary (preantral), antral, then the mature (Graafian) follicle of 18–25 mm.
- About 10 antral follicles (2–9 mm) start each cycle. By about day 7 one becomes dominant: it has the most FSH receptors and aromatase, makes the most oestradiol, and so survives as rising oestradiol and inhibin lower FSH; the others become atretic.
- The primary oocyte is held in prophase I by high cAMP supplied by granulosa cells through gap junctions; removed from the follicle it resumes meiosis. The LH surge releases the brake: meiosis I is completed just before ovulation (first polar body), and the oocyte arrests at metaphase II until fertilisation.
Ovulation
- LH surge begins 34–36 hours before ovulation and peaks 10–12 hours before it. A smaller FSH surge expands the cumulus (hyaluronic acid secretion).
- Rupture needs proteolytic enzymes and prostaglandins; high-dose NSAIDs can cause a luteinised unruptured follicle.
- Mittelschmerz: mid-cycle iliac fossa pain from follicular fluid or blood irritating the peritoneum. On scan after ovulation: the follicle has gone, a collapsed crenated cyst remains and there is fluid in the pouch of Douglas.
- Mid-cycle spotting follows the brief fall in oestradiol at ovulation.
- The fimbriae sweep the oocyte and cumulus into the tube. The ovum can be fertilised for only about 12–24 hours; sperm survive up to 5 days, so the fertile window is the 5 days before ovulation plus that day.
- Fertilisation occurs in the ampulla; the embryo reaches the uterus in 3–4 days and is fed by glycogen-rich gland secretions until it implants.
Corpus luteum
- Luteinised granulosa and theca cells secrete progesterone and oestradiol (and inhibin A).
- It has a built-in lifespan of about 14 days; luteolysis occurs unless hCG rescues it. A short luteal phase (about 8–9 days of raised temperature) suggests inadequate luteal function.
Endometrial phases
| Phase | Hormone | Histology | Ultrasound |
|---|---|---|---|
| Menstrual (days 1–5) | Progesterone withdrawal | Fragmented glands, collapsed stroma, fibrin thrombi, haemorrhage, neutrophils | Thin, irregular |
| Proliferative | Oestrogen | Straight tubular glands with mitoses; stroma grows | Trilaminar (triple-line) pattern |
| Secretory | Progesterone | Subnuclear vacuoles (first sign of ovulation, about day 17), then luminal secretion, coiled glands, stromal oedema, predecidual change | Uniformly echogenic (glycogen and secretions) |
- Mechanism of menstruation: progesterone falls, prostaglandin F2-alpha and endothelin cause spasm of spiral arteries, ischaemia breaks down the functional layer, and the basal layer remains to regenerate it; new surface epithelium grows from the basal glands even before the endometrium thickens.
- Normal loss is about 30–40 mL; more than 80 mL is the traditional threshold for heavy bleeding. Endometrial fibrinolytic activity keeps menstrual blood liquid; clots appear only when flow overwhelms it.
- Anovulatory cycles (adolescence, perimenopause) give unopposed oestrogen with irregular, heavy breakthrough bleeding. Primary dysmenorrhoea begins once cycles become ovulatory, because the secretory endometrium makes prostaglandins.
Markers of the phase
| Marker | Oestrogen (pre-ovulatory) | Progesterone (luteal) |
|---|---|---|
| Cervical mucus | Abundant, clear, stretchy (spinnbarkeit), ferning from high water and sodium chloride | Scanty, thick, no ferning |
| Cervix | Soft, high, os open | Firm, low, os closed |
| Vaginal smear | Flat superficial cells with pyknotic nuclei | Folded, clumped intermediate cells |
| Basal body temperature | Lower | Rises 0.3–0.5 °C (thermogenic action on hypothalamus) |
- Progesterone also stimulates ventilation, lowering PaCO2 in the luteal phase.
- A temperature rise lasting beyond about 18 days without a period suggests pregnancy. Home monitors detect urinary pregnanediol glucuronide.
- Premenstrual syndrome reflects sensitivity to normal hormone changes; it disappears when the cycle is suppressed.
- Practical timing: hysterosalpingography after menses and before ovulation; breast MRI in the second week, when breast enhancement is least.
Remember The luteal phase is fixed at 14 days: count back from the next period to find ovulation.
Exam trap A longer cycle lengthens the follicular phase, not the luteal phase.
Exam trap Day 21 progesterone only fits a 28-day cycle; test 7 days before the expected period.
Test yourself on this topic
5 of the 48 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
An oocyte is fertilised in the ampulla on the day of ovulation. How long does the embryo usually take to pass through the tube and reach the uterine cavity?
Answer: About 3–4 days
After fertilisation in the ampulla, the embryo is moved toward the uterus by ciliary beating and smooth muscle contraction of the tube. Transit takes about 3–4 days; it is held for a while at the ampullary-isthmic junction and then passes the isthmus as progesterone relaxes it. It reaches the cavity as a morula or early blastocyst around day 4–5 and implants at about day 6–7, when the endometrium is receptive. Delayed transit, for example from tubal damage, favours ectopic implantation.
Tubal transit
- Fertilised in the ampulla
- Transit about 3–4 days
- Arrives as morula or blastocyst
- Implants day 6–7
Remember Three or four days down the tube.
Exam trap Slow transit from tubal damage lets the blastocyst implant in the tube.
Why the other options are wrong
- About 12 hours Too short for transit through the tube.
- About 7–8 days Implantation, not arrival, occurs about 6–7 days after fertilisation.
- About 14 days Far too long.
- About 21 days Far too long.
A vaginal smear is taken from the lateral wall on day 22 of a normal ovulatory cycle. The cells are mostly folded, clumped squamous cells with vesicular nuclei rather than flat cells with pyknotic nuclei. Which cell type dominates this smear?
Answer: Intermediate cells
Vaginal squamous epithelium matures in response to hormones. Oestrogen drives full maturation to large, flat superficial cells with small pyknotic nuclei, which peak around ovulation. Progesterone in the luteal phase stops maturation at the intermediate layer, so the smear shows mainly intermediate cells, often folded and clumped, with vesicular nuclei. Parabasal cells dominate when oestrogen is absent, as in atrophy after menopause.
Vaginal maturation
- Oestrogen: superficial cells
- Progesterone: intermediate cells
- No oestrogen: parabasal cells
- Smear from lateral vaginal wall
Remember The vaginal smear reads the hormone of the day.
Exam trap Folded, clumped intermediate cells are a progesterone effect, also seen in pregnancy.
Why the other options are wrong
- Superficial cells Peak around ovulation under oestrogen.
- Parabasal cells Dominate in oestrogen deficiency.
- Basal cells Not normally shed in a smear.
- Endocervical columnar cells Glandular cells from the endocervix, not vaginal squamous cells.
A woman on a long-acting GnRH agonist has very low FSH and LH. Ovarian ultrasound still shows normal numbers of 2–4 mm antral follicles, but none grows beyond about 5 mm. At what stage does follicle growth become dependent on FSH?
Answer: Early antral stage
Activation of primordial follicles and growth through the primary and preantral stages are controlled mainly by local factors inside the ovary, so they continue even when gonadotrophins are suppressed. From the early antral stage, at roughly 2–5 mm, further growth, recruitment and selection require FSH. That is why women on GnRH agonists or the combined pill keep small antral follicles but do not develop dominant ones. AMH, made by these small follicles, also stays measurable.
When follicles need FSH
- Primordial to preantral: local control
- Early antral (2–5 mm): needs FSH
- Dominant follicle: FSH then LH
- Suppression: small follicles remain
Remember Small follicles grow alone; antral ones need FSH.
Exam trap Gonadotrophin suppression does not destroy the follicle reserve; it halts antral growth.
Why the other options are wrong
- Primordial stage Activation is gonadotrophin-independent.
- Primary follicle stage Growth at this stage does not need FSH.
- Preantral (secondary) stage Preantral growth is mainly under local control.
- Pre-ovulatory stage only FSH dependence starts well before this.
For the first 2–3 days after it enters the uterus, before it implants, a blastocyst is nourished by the uterine cavity. What does this nourishment consist of?
Answer: Glycogen-rich uterine gland secretions
Under progesterone in the secretory phase, endometrial glands become tortuous and secrete a fluid rich in glycogen, glycoproteins, lipids and growth factors into the cavity (histotroph). The free blastocyst and, in early implantation, the embryo rely on this uterine milk before the placental circulation is established. Human oocytes contain very little yolk, and maternal blood does not reach the intervillous space freely until about 10–12 weeks.
Histotroph
- Progesterone makes glands secrete
- Rich in glycogen, glycoproteins
- Nourishes the free blastocyst
- Bridges until placental circulation
Remember Before the placenta, the glands feed the embryo.
Exam trap Early pregnancy nutrition is histotrophic, not via maternal blood.
Why the other options are wrong
- Maternal blood from eroded spiral arteries Maternal blood flow to the placenta is established much later.
- Amniotic fluid secreted by the amnion The amnion has not yet formed.
- Yolk stored within the oocyte Human oocytes have little yolk.
- Oxygenated mucus from the cervix Cervical mucus does not nourish the blastocyst.
Mid-cycle cervical mucus spread on a glass slide and allowed to dry shows a fern-like crystalline pattern, which is absent in luteal-phase mucus. What property of the mucus produces the ferning?
Answer: High water and sodium chloride content
Under high oestradiol just before ovulation, the cervix secretes copious, thin, watery mucus with a high water and sodium chloride content and relatively low protein. As it dries, the sodium chloride crystallises along the glycoprotein strands into a fern pattern. Progesterone after ovulation makes the mucus scanty, thick and cellular with less salt, so ferning disappears. Ferning is therefore a bedside sign of oestrogen dominance.
Ferning
- High oestradiol
- Mucus rich in water and NaCl
- Salt crystallises on drying
- Lost under progesterone
Remember Ferning means salt in oestrogenic mucus.
Exam trap Ferning is also used to detect amniotic fluid, which is salty too.
Why the other options are wrong
- High glycogen content Glycogen is in vaginal epithelium, not the cause of ferning.
- High progesterone concentration Progesterone abolishes ferning.
- High immunoglobulin content Not the cause of the pattern.
- Abundant neutrophils and cell debris Cellular mucus is typical of the luteal phase.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Reproduction, pregnancy and labour, Menstrual cycle: ovarian and endometrial phases; ovulation
Read next
Drug dose pages
Doses, cautions and practical tips for drugs in this chapter.