The menstrual cycle starts in females at puberty and continues until menopause. In the first half of the 28 day cycle, a follicle develops in the ovary and the lining of the uterus (the endometrium) is prepared for possible implantation. Ovulation (release of an oocyte) occurs mid-cycle. If no implantation occurs, the endometrium breaks down and menstruation occurs. The cycle is controlled by four hormones. Two are secreted from the pituitary gland and two are secreted by the ovaries. Pituitary hormones: FSH and LH. Ovarian hormones: Estrogen and progesterone. The cycle involves both negative and positive feedback. Negative feedback mechanisms act against a change. Positive feedback mechanisms will reinforce a change.

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