A normal menstrual cycle involves complex hormonal interactions that stimulate the ovaries to produce and release estrogen, progesterone and ova (eggs). Menstruation normally occurs in the first five days of the cycle as a result of a decline in estrogen and progesterone levels when pregnancy does not occur. Without these hormones to support the uterine lining, it pulls away, tearing capillaries and causing bleeding.

In the follicular phase (Days 6 to 14), a shift in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) triggers production of estrogen from the ovaries, which encourages the growth of a new uterine lining, known as the endometrium. At the midpoint of the cycle, rising estrogen levels cause LH levels to surge, and in response, the ovaries release an egg, typically between Days 12 and 14.

In the next phase, Days 14 to 28, the site of egg release (corpus luteum) secretes copious amounts of progesterone, necessary to prepare the endometrial lining to receive a fertilized egg. At this stage in the luteal phase of the cycle, progesterone levels exceed estrogen levels. If pregnancy does not occur, progesterone and estrogen levels drop, leading to menstruation, and the cycle begins again.

The cycle continues until an average age of 52 and would normally only be interrupted by pregnancy and breastfeeding. Hormonal problems, including imbalances in the estrogen/progesterone ratio, can interfere with normal cycles. Problems with menstruation include amenorrhea (absent periods), menorrhagia (excessive bleeding), dysmenorrhea (extremely painful menstruation) and irregular periods. There is often a variety of underlying conditions that prompt these symptoms.