However, the study authors also stressed that the odds of a clot remain very low for any one woman using the medications, and remain lower than the clotting risk that occurs when a woman is pregnant.
“One has to weigh the benefits of using oral contraceptive pills against the risks, such as unwanted pregnancies and abnormal uterine bleeding with resultant anemia,” said Dr. Mamta Mamik, assistant professor of obstetrics, gynecology and reproductive science at the Icahn School of Medicine at Mount Sinai in New York City.
In the study, a team led by Yana Vinogradova, of the University of Nottingham in England, analyzed two large U.K. patient databases. The investigators looked specifically at blood clot risk among women aged 15 to 49 taking birth control pills.
The researchers reported that women who used pills with newer types of progestogen hormone—drospirenone, desogestrel, gestodene, and cyproterone—were 1.5 to 1.8 times more likely to develop blood clots than those who used pills containing older progestogens such as levonorgestrel, norethisterone and norgestimate.
The study wasn’t designed to prove a cause-and-effect relationship. However, compared to women who did not use birth control pills, those who used the newer pills were four times more likely to develop blood clots, and those who used the older pills were 2.5 times more likely to develop blood clots, the investigators found.
Still, the risk to any one woman remained low. In absolute terms, the extra number of blood clot cases per year per 10,000 women was six for women using birth control pills with levonorgestrel and norgestimate, compared to 14 for those using newer pills with desogestrel and cyproterone, the researchers said.
The findings were published May 26 in the BMJ.
Vinogradova’s team emphasized that birth control pills are safe, and pointed out that even the threefold increased risk of blood clots associated with birth control pills is still lower than the 10-fold increased risk that a woman experiences when she is pregnant.
Another expert in the United States agreed that birth control medications always come with pluses and minuses.
“Not only do women have effective pregnancy protection, they may also enjoy the benefits of lighter and predictable menses,” said Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. “Yet risks of oral contraceptives should also be considered,” she added.
In the new study, “the pills with higher risks include Yaz, Yasmin and Desogen,” Wu said. She believes that “doctors will need to consider this when starting a patient on oral contraceptives and considering the different brands.”