This could be the menstrual phase, when uterine lining sheds to start a fresh. Or it could be the fertile phase following menstruation.
if it is fertilized by sperm it becomes a zygote that embeds itself into the lining on uterus The placenta, which is especially prepared for this event once a month if it is not fertilized it breaks down after a couple of days which signals for the uterus to dispose of the lining in what is seen as menstrual bleeding.
It is controlled by the uterus. That is where the uterus lining is which causes the menstrual cycle
About once a month, the uterus grows a new, thickened lining (endometrium) that can hold a fertilized egg. When there is no fertilized egg to start a pregnancy, the uterus then sheds its lining. This is the monthly menstrual bleeding (also called menstruation or menstrual period). The MC is measured from the first day of menstrual bleeding, Day 1, up to Day 1 of your next menstrual bleeding. Although 28 days is the average cycle length, it is normal to have a cycle that is shorter or longer.
Spotting before your period is just very light menstrual flow, it's the process of the uterus lining shedding starting - once it gets under way the bleeding becomes heavier.
At the beginning of the menstrual period.
The uterus lining deteriorates during menstruation. The menstrual flow is the uterine lining that is shedding after pregnancy does not occur.
The uterus has 3 layers: Endometrium. This is the inner lining. It is shed during your period.
endomoetrium
condition of the inner lining of the uterus
Menstrual cycle.
endomoetrium
Menstruation is the process in which the uterus lining breaks down and leaves the body - thus menstrual flow consists of uterus lining and blood from the uterus lining breaking down, it can also contain cervical mucus picked-up while the menstrual flow passes through the vaginal opening, and discharge from within the vagina.