The purpose of the menstrual cycle is to

Answer to your question is expel an unfertilized egg and endometrium. Menstrual cycle is natures way of getting body ready for future pregnancy. it occures every 21-35 days and it lasts from 3-5 days. Menstrual cycle has it's Phases:Menstrual phase,Follicular phase,Ovulation phase,Luteal phase and The purpose of the menstrual cycle is to expel an unfertilized egg and endometrium which prepares body for pregnancy.

I KNOW TMI

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It's speed of rotation.