What typically triggers the start of menstruation?

• A. Decrease in estrogen
• B. Decrease in progesterone
• C. Increase in luteinizing hormone
• D. Increase in follicle-stimulating hormone

Answer: (B)

The onset of menstruation is primarily triggered by a decrease in progesterone levels after the corpus luteum degenerates. Following ovulation, progesterone helps to prepare the uterine lining for potential implantation of a fertilized egg. If fertilization does not occur, the levels of progesterone drop significantly. This decline signals the body that it is time to shed the uterine lining, leading to menstruation.

In this context, while estrogen levels also play a role in the menstrual cycle and can fluctuate, it is the decrease in progesterone that is the direct trigger for the physiological changes that lead to menstruation. The other hormones mentioned, such as luteinizing hormone and follicle-stimulating hormone, are more involved in regulating the menstrual cycle rather than directly initiating menstruation.