Infradian rhythms: menstrual cycle

• The menstrual cycle is an infradian rhythm that is governed by the monthly changes in hormone levels involved in ovulation.
• On average the cycle lasts 28 days, though anything between 21 and 35 days is generally considered ‘normal’ in adults.
• In young women it is not unusual to have a cycle of up to 45 days.
• Day 1 of the cycle is the first day of ‘bleeding’ (when the thickened lining of the womb starts to be shed).
• During the menstrual cycle, rising levels of the hormone oestrogen cause the ovary to develop and then release an egg (ovulation).
• After ovulation, the hormone progesterone helps the uterus lining to grow thicker, readying the body for pregnancy.
• • If pregnancy does not occur, the egg is absorbed into the body and the womb lining comes away and leaves the body.

  • This is known as the menstrual flow.
• The menstrual cycle is an endogenous system.
• However, evidence suggests that is may be influenced by exogenous factors (external to the individual).