Table 1 shows the concentration of progesterone in a woman's blood plasma during a standard non-pregnant menstrual cycle and a cycle during which fertilisation and successful implantation occur.
| Days in menstrual cycle | Progesterone level in non-pregnant cycle (nmol/L) | Progesterone level in pregnant cycle (nmol/L) |
|---|---|---|
| 1–13 | 1.1 | 1.1 |
| 14–22 | 26.8 | 27.5 |
| 23–28 | 5.3 | 62.4 |
Explain why the progesterone concentration remains high and continues to rise after day 22 in the pregnant cycle.
