The human reproductive system operates on a monthly cycle, preparing the body for a potential pregnancy. Each cycle involves the release of a mature egg from an ovary, a process known as ovulation. Not every egg released becomes fertilized. When fertilization does not occur, a series of biological events unfold to reset the cycle for the following month.
The Egg’s Journey and Lifespan
Ovulation marks the release of a mature egg from one of the ovaries. Tiny, finger-like projections called fimbriae, located at the end of the fallopian tube, gently sweep the egg into the tube. From there, muscular contractions within the fallopian tube walls slowly push the egg towards the uterus.
An egg has a limited window of viability for fertilization, typically surviving for only 12 to 24 hours after its release. If sperm are not present in the fallopian tube to fertilize the egg within this brief timeframe, the egg begins to disintegrate. It is then naturally reabsorbed by the body.
The Body’s Hormonal Response
Following ovulation, the ruptured follicle in the ovary transforms into a temporary structure called the corpus luteum. This corpus luteum begins producing progesterone and some estrogen. The primary role of these hormones is to prepare the uterine lining, or endometrium, by thickening it and enriching it with blood and nutrients, creating an environment suitable for a fertilized egg to implant.
If fertilization does not occur, the corpus luteum has a lifespan of approximately 10 to 14 days. After this period, it degenerates, eventually forming a scar tissue known as the corpus albicans. This degeneration leads to a decline in progesterone and estrogen levels. The drop in these hormone levels signals to the uterus that a pregnancy has not taken place, initiating the next phase of the menstrual cycle.
The Menstrual Phase
The decrease in progesterone and estrogen levels, caused by the degeneration of the corpus luteum, triggers a biological response in the uterus. The thickened uterine lining, which was prepared for a potential pregnancy, is no longer needed. This lining begins to break down and shed.
The shedding process involves the expulsion of this uterine tissue, along with blood and mucus, through the cervix and out of the body via the vagina. This visible process is known as menstruation. The start of this shedding marks day one of a new menstrual cycle, as the body begins its preparations for another potential ovulation.