A menstrual cycle is the process a woman's reproductive system goes through each month to release an egg from the ovaries and make a pregnancy possible.
A menstrual period (menstruation) is the monthly bleeding from the vagina that occurs at the beginning of each menstrual cycle. During menstruation, the lining of the uterus comes off and is released as menstrual blood.
In the monthly menstrual cycle:
An egg matures and is released (a process called ovulation)
The lining of the uterus swells up with blood vessels so it's ready to accept an egg that has been fertilized by a man's sperm
A fertilized egg attaches to the lining of the uterus and starts growing OR
The egg doesn't get fertilized, so it doesn't attach, and the lining of the uterus comes off and is released as menstrual blood
The days of a menstrual cycle are counted from the first day of one menstrual period to the start of the next menstrual period. A menstrual cycle usually lasts between 25 and 35 days but can vary from month to month. Usually, a menstrual period lasts 3 to 7 days.