The two halves together
The ovary's two phases and the uterus's three run on the same 28 days. Set side by side, each explains the other.
| Days | In the ovary | In the uterus | Hormone in charge |
|---|---|---|---|
| 1–4 | Follicular phase begins | Menstrual phase | progesterone falling; FSH |
| 4–14 | Follicle grows to the Graafian follicle | Proliferative phase | FSH; oestrogen |
| ~14 | Ovulation | Wall rebuilt | LH surge |
| 14–28 | Luteal phase; corpus luteum | Secretory phase | progesterone |
Body temperature
During the secretory phase, body temperature also rises slightly — by a fraction of a degree, between about 36 °C and 37 °C. It falls again as the next menstrual phase begins.
Figure to be added
The whole cycle turns on one thing
Every event after day 14 is waiting for an answer to a single question: did fertilisation happen? If it did not, the corpus luteum degenerates, progesterone falls, the uterine wall breaks down and the cycle starts again. If it did, the next section takes over.