Answer:
Progesterone promotes fertility because its levels rise during ovulation and immediately after.
Step-by-step explanation:
Progesterone is an endogenous steroid hormone that is produced by ovaries, placenta and the adrenal glands. It's main function is to maintain the lining of the uterus (endometrial lining) every month. Soon after the ovulation, progesterone is released by corpeus luteum, it levels increases in the blood. And if an egg is fertilized, the corpeus luteum will continue to produce progesterone which will stimulate blood vessels to supply to endometrium, so that a fertilized egg can be implanted successfully. And if an egg is not fertlized, the corpus luteum will breakdown and there will be decrease in progesterone level. This will cause the endometrial lining to break down and eventually menstrual cycle will begin.
Once an egg is implanted, progesterone is responsible for stimulating endometrium to provide nutrients to the developing embryo. Progesterone maintains the thickness of the endometrium throughout the pregnancy. So in short, progesterone is required for a pregnancy to proceed smoothly throughout the nine months.