Mirena is a type of intrauterine device (IUD) that provides long-acting reversible contraception. This small, T-shaped device is inserted into the uterus, offering a reliable method of birth control. Its primary function is to prevent pregnancy.
The Device and Its Active Ingredient
Mirena consists of a small, flexible T-shaped plastic frame, measuring 32 millimeters in both horizontal and vertical directions. This device is placed inside the uterus by a healthcare professional. The active ingredient in Mirena is levonorgestrel, a synthetic progestin hormone. The device is designed to release a continuous, low dose of this hormone directly into the uterine cavity, with an initial release rate of approximately 20 micrograms per day.
How Pregnancy is Prevented
The levonorgestrel released by Mirena employs several mechanisms to prevent pregnancy. One key action involves thickening the cervical mucus, creating a physical barrier that impedes sperm from entering the uterus and reaching an egg.
The hormone also causes a thinning of the uterine lining, known as the endometrium. This alteration makes the uterine environment less hospitable for a fertilized egg to implant.
The local presence of levonorgestrel within the uterus and fallopian tubes affects sperm. It inhibits sperm movement and survival, reducing their ability to reach and fertilize an egg.
While not its primary mechanism, Mirena can also suppress ovulation in some individuals. In initial studies, approximately 45% of menstrual cycles were ovulatory in the first year, with this percentage increasing to 75% after four years. This partial or full suppression of egg release provides an additional layer of protection against pregnancy.
Mechanism for Reducing Bleeding
Mirena is also effective in reducing heavy menstrual bleeding. The local release of levonorgestrel directly affects the uterine lining. This hormone causes the endometrium to become thin and inactive, reducing its monthly development.
The reduction in endometrial thickness and activity leads to lighter and shorter menstrual periods. Many women experience a significant decrease in blood loss, and some may even achieve amenorrhea, which is the absence of periods, over time. Clinical trials have shown an 80% reduction in bleeding after three months and over 90% reduction after six months for women with heavy menstrual bleeding. This effect on the uterine lining is a distinct, therapeutic action of Mirena.