Menstrual cycle occurs only in women. It can start from when they are young and ends only when they attain their menopausal stage. Menstruation is said to be the crying of the uterus when pregnancy didn’t happen in a woman’s body every month.
Menstrual cycle is the bleeding from the crumbling of the lining of the uterus which entails the culmination of a series of interlocked events that prepares the uterus to accept a fertilized egg. So when pregnancy did not take place in a woman, this prepared lining is shed from the uterus causing the entire cycle of events to take place.