Physiology chapters
Menstrual cycle, pregnancy and lactation
The phases of the ovarian and uterine cycles and the hormones that drive them, how the LH surge triggers ovulation, the hormones and body changes of pregnancy, and how milk is made and released.
- The cycle (about 28 days) has a follicular phase (FSH, rising oestrogen, proliferative endometrium) and a luteal phase (corpus luteum, progesterone, secretory endometrium); the luteal phase is fixed at about 14 days.
- High oestrogen switches to positive feedback, causing the LH surge; ovulation follows about 16–36 hours later (typically 10–12 hours after the LH peak).
- In pregnancy, hCG keeps the corpus luteum going until the placenta takes over; in lactation, prolactin makes milk and oxytocin ejects it.
On this page
Overview
The female reproductive cycle prepares an egg for release and the uterine lining for implantation each month. It is controlled by the hypothalamus (GnRH), pituitary (FSH, LH) and ovaries (oestrogen, progesterone, inhibin).
If pregnancy occurs, the placenta takes over hormone production. After birth, prolactin and oxytocin control lactation.
The ovarian cycle
Follicular phase (day 1 to ovulation)
- FSH rises at the start of the cycle and stimulates a group of follicles to grow.
- Two-cell model: theca cells respond to LH and make androgens; granulosa cells respond to FSH and convert the androgens to oestrogen with aromatase.
- By about day 5–7, one dominant follicle is selected; the others regress (atresia).
- Oestrogen rises steadily. At first it inhibits FSH and LH (negative feedback), helped by inhibin.
The follicular phase varies in length; this is why cycle length varies.
Ovulation (about day 14 of a 28-day cycle)
- When oestrogen stays high (above a threshold for about 2 days), its feedback switches to positive.
- The pituitary releases a large LH surge (and a smaller FSH surge).
- Ovulation follows, typically about 10–12 hours after the LH peak (16–36 hours after the start of the surge).
- The LH surge also causes the primary oocyte to complete meiosis I.
The LH surge is triggered by high oestrogen acting by positive feedback. It causes ovulation.
Luteal phase (ovulation to menstruation)
- The ruptured follicle becomes the corpus luteum, which makes progesterone (and oestrogen and inhibin).
- Progesterone and oestrogen suppress FSH and LH.
- If no pregnancy occurs, the corpus luteum degenerates after about 12–14 days into the corpus albicans.
- Progesterone and oestrogen fall; FSH rises again and a new cycle starts.
The luteal phase is fairly constant (about 14 days). Ovulation therefore happens about 14 days before the next period, not necessarily on day 14.
The uterine (endometrial) cycle
| Phase | Days (28-day cycle) | Hormone | Endometrium |
|---|---|---|---|
| Menstrual | 1–5 | Fall in progesterone and oestrogen | Lining sheds (spiral arteries constrict; prostaglandins) |
| Proliferative | 6–14 | Oestrogen | Regrows; glands and stroma multiply |
| Secretory | 15–28 | Progesterone | Glands coil and secrete glycogen-rich fluid; stroma oedematous; ready for implantation |
Oestrogen → proliferative phase. Progesterone → secretory phase. Menstruation follows the fall in progesterone.
Actions of oestrogen and progesterone
| Oestrogen | Progesterone |
|---|---|
| Growth of the uterus, vagina, breasts (ducts) | Secretory endometrium; maintains pregnancy |
| Proliferative endometrium | Thick, scanty cervical mucus (hostile to sperm) |
| Thin, stretchy cervical mucus at ovulation (spinnbarkeit) | Relaxes uterine muscle |
| Female fat distribution; closure of epiphyses | Raises basal body temperature by about 0.3–0.5°C after ovulation |
| Raises HDL; protects bone | Breast lobule and alveolar development |
| Raises binding globulins (TBG, SHBG) | Increases breathing (lowers PCO2) in pregnancy |
The rise in basal body temperature after ovulation is used to confirm that ovulation has occurred.
Pregnancy
Hormones
| Hormone | Source | Role |
|---|---|---|
| hCG | Syncytiotrophoblast | Acts like LH; keeps the corpus luteum making progesterone in early pregnancy; peaks at about 8–10 weeks; basis of pregnancy tests |
| Progesterone | Corpus luteum, then placenta (from about 8–12 weeks) | Maintains the endometrium (decidua); relaxes the uterus |
| Oestrogens (mainly oestriol) | Placenta (with fetal adrenal and liver) | Growth of uterus and breasts; relaxes pelvic ligaments |
| Human placental lactogen (hPL) | Placenta | Anti-insulin: raises maternal glucose for the fetus; breast development |
| Relaxin | Corpus luteum, placenta | Softens the cervix and pelvic ligaments |
hCG rescues the corpus luteum until the placenta can make enough progesterone (luteal-placental shift at about 8–12 weeks).
Physiological changes in the mother
| System | Change |
|---|---|
| Blood | Plasma volume ↑ about 40–50%; red cell mass ↑ less (about 20–30%) → physiological (dilutional) anaemia; clotting factors ↑ (hypercoagulable); white cells ↑ |
| Heart | Cardiac output ↑ about 30–50% (rate and stroke volume); peripheral resistance ↓; BP falls slightly in mid-pregnancy |
| Lungs | Tidal volume and minute ventilation ↑ (progesterone); PCO2 falls (mild respiratory alkalosis); FRC ↓ |
| Kidneys | GFR ↑ about 50%; creatinine and urea fall; glycosuria more common |
| Metabolism | Insulin resistance in later pregnancy (hPL, cortisol); weight gain about 10–12 kg |
| Gut | Slower motility (constipation), reflux |
| Thyroid | Total T4 ↑ (TBG ↑); free T4 normal |
A haemoglobin that looks mildly low in pregnancy may be dilutional, because plasma volume rises more than red cell mass.
Lactation
- In pregnancy, oestrogen and progesterone develop the breast ducts and alveoli; prolactin rises, but high oestrogen and progesterone block milk production.
- After delivery, these hormones fall sharply, so prolactin can start milk production within a few days.
- Suckling stimulates nerves in the nipple, which:
- Increase prolactin (by reducing dopamine): milk production for the next feed.
- Release oxytocin from the posterior pituitary: milk ejection (let-down) by contracting myoepithelial cells.
- Oxytocin release can be triggered by the baby's cry and inhibited by stress.
High prolactin inhibits GnRH, so ovulation is suppressed during frequent breastfeeding (lactational amenorrhoea). This is not fully reliable as contraception.
Prolactin → milk production. Oxytocin → milk ejection. Suckling stimulates both.
Clinical relevance
Combined oral contraceptive pill
How it works: oestrogen and progestogen given together suppress FSH and the LH surge by negative feedback, so ovulation does not occur. The progestogen also thickens cervical mucus.
Menopause
How it happens: the ovaries run out of follicles, usually around 45–55 years.
What you see: oestrogen falls, so FSH and LH rise (FSH especially); hot flushes, vaginal dryness and bone loss.
Expect: the hormones of each phase, the two-cell model, positive feedback and the LH surge, the fixed luteal phase, oestrogen vs progesterone actions (cervical mucus, temperature), hCG and the corpus luteum, maternal changes in blood, heart, lungs and kidneys, and prolactin vs oxytocin in lactation.
Quick revision
- Follicular phase: FSH, oestrogen, proliferative endometrium; variable length.
- High oestrogen → positive feedback → LH surge → ovulation.
- Luteal phase: corpus luteum, progesterone, secretory endometrium; about 14 days.
- Progesterone raises basal temperature and thickens cervical mucus.
- hCG keeps the corpus luteum alive until the placenta takes over.
- Pregnancy: ↑ cardiac output, ↑ GFR, dilutional anaemia, ↓ PCO2.
- Prolactin makes milk; oxytocin ejects it.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What causes ovulation?
The LH surge, triggered by high oestrogen levels switching to positive feedback on the pituitary.
Which phase of the menstrual cycle has a fixed length?
The luteal phase, which lasts about 14 days.
What maintains the corpus luteum in early pregnancy?
Human chorionic gonadotrophin (hCG) from the syncytiotrophoblast.
What is the role of oxytocin in lactation?
It contracts the myoepithelial cells of the breast to eject milk (the let-down reflex).