The menstrual cycle is the period in which the female body prepares for fertilisation.
The cycle is controlled by hormones (LH and FSH). The length of the cycle varies depending on the species (rat 5 days, woman 28 days), begins at puberty and ends at the menopause.
Its culminating point is ovulation. The uterus prepares to receive the fertilised ovum and the egg and possibly the embryo by thickening and developing a richer blood vessel supply. If fertilisation and embryo implantation does not occur, the uterine epithelium is lost through a bleed (period or menstruation) and a new cycle begins.
If fertilisation occurs, the cycle does not finish and a pregnancy begins.