Pregnancy happens when an oocyte, or egg from the ovary, is released and is fertilized by spermatozoa, soon to be implanted in the uterine lining. The oocyte is connected to a follicle, which becomes the corpus luteum, and which regresses if pregnancy does not occur.
When the oocyte is fertilized, human chorionic gonadotropin, the pregnancy hormone, stimulates the corpus luteum to produce progesterone until there is a shift from the corpus luteum to placental production of progesterone, which takes about 5 weeks. | | | | |