The average menstrual cycle lasts 28 days, but it can vary from 20 to 45 days. The average cycle can be split into two phases – the follicular phase (days 1 to 14) and luteal phase (days 15 to 28).
The follicular phase begins with the shedding of the uterus lining (menstrual bleeding) which marks day 1 of the cycle and typically lasts about 5 days. This is followed by the reproduction of the endometrium (uterus lining) in preparation for an egg.
The luteal phase begins with the formation of the corpus luteum (a structure that develops in the ovary each cycle that secretes reproductive hormones), ending with either pregnancy or degradation of the corpus luteum on approximately day 28.
The reproductive hormones, primarily oestrogen and progesterone, are the key factors involved in the control of this system and are regulated by a feedback loop involving two other hormones – luteinizing hormone and follicle stimulating hormone.