What happens at the beginning of the menstrual cycle?

What happens if the ovum fails to get fertilized?

Changes in $GnRH$ pulse frequency in females control the secretion of ..........

Which hormone regulates the human menstrual cycle?

The corpus luteum in pregnancy has a long life. However,if fertilization does not take place,it remains active only for $10-12$ days. Explain.

The following graph shows the relative concentrations of the four hormones present in the blood plasma of a woman during her menstrual cycle. Identify the hormones $A, B, C$ and $D$.