To understand the cause of periods, you need an in-depth understanding of the menstrual cycle. Periods involve the discharge of blood from the womb lining. Each month, during the follicular phase, this lining develops to potentially receive a fertilised egg.
During the luteal phase, if sperm does not fertilise the egg, it does not implant in the uterus. Once the body realises this, oestrogen and progesterone levels drop, causing the surface layer of the endometrium to detach and be expelled through the vagina, thus marking the start of menstruation and, at the same time, a new menstrual cycle.
Between puberty and menopause, except during periods of pregnancy or breastfeeding, periods are expected to occur regularly, on average every 28 days (with a range of 26 to 31 days). Generally, periods last between 2 and 6 days.

