Introduction
The menstrual cycle is controlled by the hypothalamic–pituitary–ovarian axis. A typical cycle lasts 28 days (normal 21–35), with bleeding for 2–7 days and a loss under 80 ml.
The axis
- Hypothalamus releases GnRH in pulses (about every 60–90 minutes); continuous GnRH would switch the pituitary off (the basis of GnRH agonist therapy).
- Pituitary releases FSH (follicle growth, oestrogen production by aromatase in granulosa cells) and LH (androgen production in theca cells, ovulation, corpus luteum).
- Ovary releases oestradiol, progesterone and inhibin, which feed back to the hypothalamus and pituitary.
- Feedback: oestrogen and progesterone normally exert negative feedback; a sustained high oestradiol (> 200 pg/ml for about 48 h) switches to positive feedback and triggers the LH surge.
Signs of ovulation
- Basal body temperature rises by 0.3–0.5 °C after ovulation (progesterone), a biphasic chart.
- Mid-luteal progesterone (day 21 of a 28-day cycle) > 30 nmol/L confirms ovulation.
- Cervical mucus clear, stretchy (spinnbarkeit) and ferning at ovulation; LH urine tests; follicle tracking on ultrasound.
- Mittelschmerz (mid-cycle pain).
The follicular phase varies in length, the luteal phase is fixed at about 14 days: ovulation is 14 days before the next period, not 14 days after the last.