The exact timing of the menarche is affected by genetic, racial, socio-economic and climatic factors. At the menarche, the duration of blood loss may be from two to nine days, but limited. The duration gradually decreases until by the age of 16 years, it lasts for an average of six days. The normal adult menstrual cycle varies from 21 to 25 days and is rarely the often-quoted 28 days. Stress can either shorten or lengthen the menstrual cycle.
All sport is a stress, and usually lengthens it.
Females who go to altitude to compete or train, have an added stress due to the lack of oxygen, which usually shortens the cycle. It is important for a sportswoman and her advisers to know what is going on in her body and at what stage the cycle is at. The ovum ripens before release in a sort of shell, the graafian follicle. The ovum is released, leaving the follicle remains behind in the ovary, and grows into a small endocrine gland, a corpus luteum. This produces a hormone, progesterone, that stimulates the uterus lining to form a thick layer with additional blood supply ready to receive an embryo if the ovum becomes fertilised.