The secretory phase: days 15 to 28
During this phase, the blood concentration of progesterone increases, which causes even more blood vessels to grow into the endometrium. This makes the endometrium receptive to the fertilised ovum. If the ovum is fertilised and the embryo implants in the endometrium and a placenta develops, it produces a hormone called human chorionic gonadotropin (HCG) throughout pregnancy. The detection of HCG in a woman's urine is the basis of most pregnancy tests.
Chorionic is pronounced 'korr ee onn ik'. Gonadotropin is pronounced 'gonn add oh troh pinn'.
HCG signals the corpus luteum to continue to supply progesterone to maintain the thick, nourishing endometrium throughout the pregnancy. Continuous levels of progesterone act as a negative feedback mechanism on the hypothalamus and pituitary gland, preventing the release of FSH and LH, and hence further ovulation ceases.
What happens if fertilisation of the ovum does not occur?
The corpus luteum degenerates and the level of progesterone declines; the endometrium disintegrates and the woman menstruates — a signal that she did not become pregnant during that menstrual cycle.