Progesterone is the hormone responsible for maintaining the uterine lining. Estrogen also plays a role in the initial thickening of the lining. Before pregnancy occurs, progesterone and estrogen levels are maintained by the corpus luteum, which is a remnant of the ruptured ovarian follicle, explains The Merck Manual Home Edition.
Ovulation occurs after a surge in luteinizing hormone, which causes the rupture of an ovarian follicle and the release of an egg. After this process, the remaining part of the follicle is called the corpus luteum. If the released egg is not fertilized, the corpus luteum degenerates and causes a drop in progesterone and estrogen. This results in the subsequent shedding of the uterine lining, known as menstrual bleeding, explains The Merck Manual Home Edition.
If fertilization does occur, the cells around the implanted embryo begin producing the hormone called human chorionic gonadotropin, which maintains the corpus luteum. The maintained corpus luteum keeps the levels of progesterone and estrogen elevated, in turn maintaining the uterine lining and preventing menstrual bleeding from occurring. Later in the pregnancy the embryo produces its own progesterone, according to The Merck Manual Home Edition.
Home pregnancy tests, as well as blood pregnancy tests, look for increased levels of human chorionic gonadotropin. Its levels rise steadily in the first 16 weeks of pregnancy, peak at around 14 weeks and begin to decline later in pregnancy, explains MedlinePlus.