Table 1 shows the concentration of progesterone measured in the blood plasma of a healthy woman at five different points during her 28-day menstrual cycle.
| Day of menstrual cycle | Progesterone concentration / nmol dm−3\text{nmol dm}^{-3}nmol dm−3 |
|---|---|
| Day 4 | 0.8 |
| Day 12 | 1.1 |
| Day 16 | 18.4 |
| Day 21 | 45.2 |
| Day 27 | 3.1 |
Explain the physiological mechanism responsible for the rapid rise in progesterone concentration between day 12 and day 21.
