Progesterone is a hormone produced in the ovaries that plays a crucial role in a woman’s menstrual cycle. It prepares the uterus for implantation and development of an embryo, and its decline leads to endometrial loss. Birth control pills containing progesterone prevent ovulation and fertilization.
Progesterone, a C-21 steroid hormone produced in the ovaries, serves as a catalyst for the onset of a woman’s periods, or menstrual cycle. Progesterone’s effect on menstruation comes from its inhibition by another hormone, gonadotropin-releasing hormone (GnRH). The average menstrual cycle lasts 28 days, and around the second to third week of the cycle, called the luteal phase, progesterone levels rise rapidly in the female system. High levels of progesterone transform the endometrium in the uterus into a hospitable lining for the implantation and development of an embryo. If implantation does not occur, GnRH suppresses progesterone through a negative feedback loop, and the decline in progesterone production results in endometrial loss.
The effect of progesterone on menstruation results in normal menstrual bleeding or progesterone withdrawal bleeding. The reduction of progesterone at the beginning of the woman’s cycle favors the expulsion of the endometrium because high levels of progesterone decrease the contractility of the uterine muscle. As the levels drop, if fertilization has not occurred, the uterus begins to contract because progesterone does not inhibit its contraction. This is part of the same mechanism that causes contractions when a woman is ready to give birth. At the end of pregnancy, progesterone levels drop, which then initiates labor.
Progesterone production in the ovaries is initiated by a third hormone associated with menstruation, called luteinizing hormone (LH). The effect of progesterone on menstruation begins when the pituitary gland begins to produce LH, which causes progesterone to rise during the luteal phase. During this phase, activated LH triggers the release of an egg from the follicle, called ovulation. Under the continued direction of LH, the eggless follicle forms in the corpus luteum, and it is the corpus luteum that is the specific generator of progesterone within the ovary. This structure continues to produce progesterone until levels are high enough to inhibit GnRH, which in accordance with the negative feedback loop causes progesterone levels to drop, inducing menstruation.
It is because of progesterone’s studied effects on menstruation that birth control pills (BCPs) are a reliable form of birth control. Most BCPs contain progesterone or progesterone-like substances that regulate a woman’s cycle and ensure that GnRH is inhibited at the right time to prevent the average LH cycle surge, which prevents ovulation. Fertilization and pregnancy are impossible in this situation because there is no egg released from the follicle that could develop into an embryo.
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