A study published in the BMJ, the online journal of the British Medical Association, adds to a growing body of evidence linking newer birth control pills to an increased risk of blood clots. Researchers found that women taking newer oral contraceptives were four times as likely to experience venous thromboembolism (VTE), or blood clot, compared to those not taking birth control pills.
These findings indicate that newer birth control pills increase the risk of blood clots just as much or more than older ones, although the risk is still very low. The study found that 14 out of 10,000 women developed VTE. By comparison, pregnant women are ten times as likely to develop a blood clot. Estrogen is the prime reason for increased risk of blood clots in women who are pregnant or taking birth control pills. “In layman’s terms, it makes the blood a little thicker,” said Dr. Barbarva Levy to NBC. “Pregnancy is a high estrogen state that really increases the risk of blood clots.” Dr. Levy is the American College of Obstetricians and Gynecologists vice president and was not involved in the study.
The increased risk of blood clots with newer birth control pills is well known among physicians. While the exact mechanism is unknown, many suspect that the increased risk is due to a second hormone called progesterone or progestin, which is present in new formulations. Levy noted that “Different progestins have different effects,” according to NBC.
The study included data from 10,000 British women, roughly half of whom were diagnosed with a blood clot. “In this observational study based on two large primary care databases, women exposed to drospirenone, gestodene, cyproterone, and desogestrel within the last 28 days had around a four times increased risk of VTE,” said the researchers, who were led by Yana Vinogradova of Britain’s University of Nottingham. “Women exposed to levonorgestrel, norethisterone, and norgestimate had about two and a half times increase in VTE risk compared with women not exposed in the past year,” Newer birth control pills include brands such as Yaz, Yasmin and Marvelon.
“The way that this study was done was not perfect and the conclusion may or may not be correct,” said Levy. “The biggest risk for blood clots is in the first six months that you take pills,”