Most tissues in your body grow only in specific areas. Endometrial tissue, which lines the inside of the uterus, is an exception to the rule, sometimes appearing in areas far from the uterus, causing a disorder called endometriosis. During your menstrual period, endometrial tissue outside the uterus builds up and then breaks down just as if it were in the uterus. Endometriosis, which affects between 2 and 10 percent of women of childbearing age, according to The Ohio State University, causes chronic pain. Progesterone-only pills prevent menstrual bleeding that worsens endometriosis.
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How Endometriosis Develops
Each month, endometrial tissue both inside and outside the uterus thickens under the influence of female hormones so an embryo can implant. If you don't get pregnant, the endometrial tissue both inside and outside the uterus is shed as menstrual bleeding. Scarring and irritation can occur in areas affected by endometriosis, which most often include the ovaries, intestines or bladder. Every time the tissue bleeds, more irritation and scarring can occur. Between 30 and 40 percent of women with endometriosis have some degree of infertility caused by damage to the ovary or fallopian tubes, according to endometriosis.org. Progesterone pills interfere with the normal hormonal cycle, preventing endometrial tissue from thickening and shedding.
Progesterone and the Menstrual Cycle
Progesterone, a female hormone responsible for changing the uterine lining after ovulation so an embryo can implant, is normally present only in small amounts in the first half of the menstrual cycle. Levels increase after ovulation and drop if you don't become pregnant. When progesterone levels drop, the uterine lining breaks down. If you take progesterone on a daily basis throughout the menstrual cycle, your ovaries don't develop an egg that month because the high progesterone levels "fool" your body into thinking it's already produced an egg that month. If you take progesterone continuously, you won't get a menstrual period that month and bleeding won't occur in areas affected by endometriosis.
Taking Progesterone Pills
Some birth control pills contain artificial progesterone, called progestin. Progestin-only pills do not contain any estrogen, the dominant female hormone. Since the ovary produces an egg each month only under the influence of estrogen, normal menstrual periods do not occur. While you will have a withdrawal bleed if you stop the progestin-only pill, the amount of bleeding is usually less than usual, because the endometrial tissue doesn't thicken as much without estrogen. Spotting often occurs in the first three to six months on the pill as your body adjusts to the changing hormone levels.
Progesterone therapy controls pain in around 75 percent of women with endometriosis, according to endometriosis.org. You can't get pregnant while you take progesterone-only pills throughout your menstrual cycle, because they prevent ovulation. Don't take progesterone while breastfeeding unless your doctor approves. Taking progesterone pills also won't decrease the damage endometriosis has already inflicted on tissues; the pills only help prevent more damage. Progesterone supplements can also cause depression, acne, bloating or weight gain. Used long-term, progestins can affect bone density; you may need bone density scans periodically.