Selecting Which Birth Control Matters

Hormonal birth control methods have different risk levels for blood clots


A new study reveals that certain non-oral types of contraception based on hormones, such as skin patches, implants or vaginal rings, do have a higher risk of blood clots.

"Discuss the possible side effects of your birth control method with your gynecologist."

Lead author Øjvind Lidegaard, a professor at the University of Copenhagen, and colleagues conducted the study looking at women's use of non-oral hormonal contraceptives and the frequency of blood clots occurring for the first time in these women.

Blood clots occur when the blood thickens and it becomes stuck in a vein. The medical term for this is venous thrombosis.

Deep vein thrombosis occurs in the lower body, usually in the legs, though the blood clot can break free and move up through the body. A pulmonary embolism is a blood clot that has migrated into the pulmonary circulation.

Lidegaard's team reviewed the data for all Danish non-pregnant women aged 15 to 49 during the time period of 2001 to 2010. None of the women had a history of blood clots or cancer (which can increase the likelihood of getting a blood clot).

They looked at information on 9,429,128 combined years of women taking birth control, which means they added together the amount of time each woman was on the birth control and added up all the women's combined time on the contraceptive form.

They found a total of 3,434 incidents of venous thrombosis in women who had not previously had a blood clot.

Among those not using any form of hormone contraception, the rate was two blood clots out of every 10,000 combined years worth of time.

Women taking a birth control pill with the hormone levonorgestrel in it had a rate of 6.2 blot clots out of every 10,000 years worth of exposure to the contraception - three times as many as those not on a hormone contraception.

Women using the vaginal ring saw 7.8 blot clots out of 10,000 years worth of exposure to the contraception, or 6.5 times the risk of not taking a hormonal contraception.

Women using the skin patch contraception that contained hormones saw 9.7 blot clot events out of 10,000 years worth of exposure, or eight times the risk of those not using hormonal birth control.

For progestogen-only IUDs, the intrauterine devices shaped like a T that are placed in a woman's cervix for months or years, the researchers found no risk of blood clots greater than what someone not taking birth control would experience. The researchers said it's possible that hormone-releasing IUDs may even have a slightly lower risk of blood clots than the risk among women not using hormone contraception.

They also found only a small, not significantly increased risk with progestogen-only implants that release hormones from just under the skin.

The rates are several times greater for hormonal birth control than non-use of hormone contraception, but the risk of a blood clot is still very small for these forms.

With the combined birth control pill, the risk of blood clot decreases over time, but this reduced risk from long-term use was not seen with the patch or vaginal ring.

The researchers concluded that slightly fewer blood clots would occur if a large number of women switched to the combined pill from the skin patch or vaginal ring. (One blood clot a year would be prevented if 2,000 vaginal ring-using women switched and 1,250 skin-patch wearing women switched.)

The study was published online May 10 in the journal BMJ. The research was funded by the Juliane Marie Centre's Gynecological Clinic in Rigshospitalet, a hospital in Denmark.

The primary investigator has received fees for speeches on pharmacology issues from various pharmaceutical companies within the past three years. One other author participated in two conferences that were paid for by pharmaceutical companies. The other authors declared no financial conflicts of interests.

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Review Date: 
May 13, 2012
Last Updated:
July 11, 2012