What triggers menstruation to occur?

The primary hormone that triggers menstruation is progesterone, which is released from the ovaries after ovulation. Progesterone prepares the uterus for implantation of a fertilized egg. If pregnancy does not occur, progesterone levels decline, causing the lining of the uterus to break down and shed, leading to menstruation.

Here is a more detailed explanation of the hormonal changes that occur during the menstrual cycle, which ultimately lead to menstruation:

1. Follicular Phase: The menstrual cycle begins with the follicular phase, which is stimulated by the release of follicle-stimulating hormone (FSH) from the pituitary gland in the brain. FSH promotes the growth and development of follicles in the ovaries. One of these follicles will mature and release an egg in a process called ovulation.

2. Ovulation: When the mature follicle ruptures and releases the egg, a surge in luteinizing hormone (LH) from the pituitary gland occurs, triggering ovulation. The released egg travels down the fallopian tube toward the uterus.

3. Luteal Phase: After ovulation, the ruptured follicle transforms into a structure called the corpus luteum. The corpus luteum produces progesterone and some estrogen. These hormones help to thicken and prepare the uterine lining (endometrium) for possible implantation of a fertilized egg.

4. Decline of Progesterone and Estrogen: If implantation does not occur, the corpus luteum begins to deteriorate, leading to a decline in progesterone and estrogen levels. This drop in hormone levels causes the uterine lining to break down and shed, leading to menstruation.

5. Menstruation: As the endometrial lining sheds and is expelled from the uterus, menstruation occurs. Menstrual blood is composed of shed uterine lining, mucus, and blood vessels. The shedding process typically lasts for several days until the uterine lining is completely expelled.

6. New Cycle Begins: After menstruation is complete, the levels of FSH and estrogen rise again, initiating the development of new follicles and preparing the body for the next menstrual cycle.