Progesterone, a C-21 steroid hormone produced in the ovaries, acts as a catalyst for the start of a woman’s menses, or menstruation cycle. The effect of progesterone on menstruation comes from its inhibition by another hormone, gonadotropin-releasing hormone (GnRH). The average menstrual cycle lasts 28 days, and at 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 covert the endometrium in the uterus into a hospitable lining for an embryo to implant and develop. If implantation does not occur, GnRH suppresses progesterone by way of a negative feedback loop, and the drop in progesterone production causes the shedding of the endometrium.
The effect of progesterone on menstruation causes normal menstrual bleeding, or progesterone withdrawal bleeding. The reduction of progesterone at the beginning of a woman’s cycle encourages the expulsion of the endometrium because high levels of progesterone decrease the contractility of the uterine muscle. As the levels fall, if fertilization has not taken place, the uterus begins to cramp because progesterone is not inhibiting its contraction. This is part of the same mechanism that brings on contractions when a woman is ready to deliver a baby. At the end of pregnancy, progesterone levels fall, which then start 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 starts producing LH, which causes the surge of progesterone during the luteal phase. During this phase, the activated LH triggers the release of an egg from the follicle, called ovulation. Under the continued direction of LH, the eggless follicle forms into 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 the levels are high enough to inhibit GnRH, which in accord with the negative feedback loop, causes progesterone levels to fall, inducing menstruation.
It is due to the studied effects of progesterone on menstruation that birth control pills (BCPs) are a trusted form of birth control. Most BCPs contain progesterone or progesterone-like substances that regulate the woman’s cycle and ensure that GnRH is inhibited at the right time to prevent the midcyle surge of LH, 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.