What starts the process of menstruation when fertilization does not occur?

The process of menstruation begins when fertilization does not occur, primarily triggered by a decrease in progesterone levels. After ovulation, the corpus luteum forms and produces progesterone, which is crucial for maintaining the uterine lining in preparation for a potential pregnancy. If fertilization does not happen, the corpus luteum eventually degenerates, leading to a drop in progesterone levels.

This decline in progesterone is significant because it causes the uterine lining to become unstable, leading to its breakdown and eventual shedding, resulting in menstruation. The decrease in progesterone signals the body that pregnancy has not occurred, prompting the cyclical shedding of the endometrial lining.

The other options, while related to the menstrual cycle, do not initiate menstruation. For instance, an increase in estrogen levels typically occurs before ovulation and is not directly responsible for the onset of menstruation. The growth of the corpus luteum is a result of the ovulation process and sustains progesterone production which, if maintained, would prevent menstruation. The release of luteinizing hormone (LH) is involved in triggering ovulation but does not play a role in the initiation of menstruation when no fertilization occurs.