Follicular phase, ovulation at the LH surge, then a fixed 14-day luteal phase
Mnemonic
Two organs, two sets of names, one cycle:
Ovarian: follicular phase, ovulation, luteal phase. Endometrial: menstrual, proliferative, secretory.
- Follicular = proliferative, driven by oestrogen from the developing follicle
- Luteal = secretory, driven by progesterone from the corpus luteum
“Oestrogen proliferates, progesterone secretes.”
The luteal phase is fixed at 14 days; all variation in cycle length comes from the follicular phase, which is why ovulation is dated backwards from the next period rather than forwards from the last.
Ovulation is triggered by the LH surge, itself caused by rising oestrogen switching from negative to positive feedback, roughly 36 hours before release.
If fertilisation does not occur the corpus luteum involutes at about day 26, progesterone falls, and the endometrium is shed. If it does, hCG from the trophoblast rescues the corpus luteum, which is what a pregnancy test detects.
Expansion
Follicular phase (variable length)
- FSH stimulates follicular development; the dominant follicle produces oestrogen
- Oestrogen thickens the endometrium (proliferative phase)
- Rising oestrogen initially suppresses LH, then above a threshold for about 48 hours switches to positive feedback
Ovulation (day 14 in a 28-day cycle)
- The LH surge triggers ovulation about 36 hours later
- Basal body temperature rises about 0.5 degrees afterwards, from progesterone
Luteal phase (fixed at 14 days)
- The corpus luteum produces progesterone, which makes the endometrium secretory
- Without fertilisation it regresses after 14 days, progesterone falls and menstruation follows
- With fertilisation, hCG from the trophoblast rescues the corpus luteum
The fixed luteal phase is the practical point: ovulation occurs 14 days before the next period, not 14 days after the last, which is how ovulation is timed in women with irregular cycles.