The Menstrual Cycle

Is defined as the periodic cyclical shedding of pro-gestational endometrium accompanied by blood flow through the vagina.

Is made up of 2 cycles.
1. Ovarian cycle: Follicular, Ovulatory and Luteal phases
2. Uterine Cycle: Menstrual, Proliferative and secretory phases

Normal values
Menstrual cycle: 28+/- 7
Menstrual Phase/menstruation:4+/-2
Blood loss:40mls+/-20
Due to variation in menstrual cycle duration, ovulation happens at different periods. In a 21 day cycle at day 7, in a 28 days cycle at 14th day and in a 35 day cycle, it occurs at day 21.
Components of normal menstruation discharge.
Endometrial cells
Blood cells
Histiocytes
Macrophages
Vaginal epithelial cells
mast cells
Prostaglandin
Fibrinolysin

Of note is that normal menstuation blood do not flow in clots except debris in it and this is due to presence of fibrinolysin as its component. Hence it is of use in differential diagnosis is menometrorrhagia.
The cause of menstruation remains multi- theoretical but the most referenced remains progesterone and estrogen deprivation.