In new recommendations, the group says that women should be aware of the "possible minimally increased risk" of blood clots with birth control pills containing the hormone drospirenone.

Those include big-selling brands like Yaz, Yasmin, Beyaz and Safyral, along with their generic equivalents.

It has long been known that women on the Pill have a small, but higher-than-average risk of blood clots. And some recent studies have suggested that the risk is relatively higher with pills containing drospirenone.

But because of "study weaknesses," it's not certain whether that is the case, said Dr. Kavita Nanda, who helped draft the new ACOG recommendations.

And the absolute risk of blood clots with any birth control pill is small, ACOG says in a report in its journal Obstetrics & Gynecology.

For every 10,000 women using birth control pills over a year, there would be anywhere from three to nine cases of blood clots called venous thromboemboli. That compares with one to five cases for every 10,000 women who are not pregnant and not on the Pill.

With drospirenone-containing pills, some studies suggest there would be 10 cases per 10,000 women each year.

Venous thromboemboli usually form in the leg veins, but can travel to the lungs, where they cause a sometimes life-threatening condition called pulmonary embolism.

Drospirenone is a synthetic form of progesterone that's different from the versions used in older "generations" of the Pill. Yasmin, which arrived on the market about a decade ago, and its sister "Yaz" products have been promoted as causing less weight gain and swelling than older-generation pills.

"Drospirenone oral contraceptives may also have advantages for some women with acne or premenstrual dysphoric disorder," Nanda said in an email, referring to a severe form of PMS that causes physical symptoms and severe mood swings.

FDA WARNING

Earlier, industry-funded studies of Yasmin, Yaz and related pills had found no elevated blood clot risk versus other Pill formulations. But several studies since 2009 have linked the newer contraceptives to relatively higher risks.

In a review of records from more than 800,000 U.S. women, the U.S. Food and Drug Administration (FDA) concluded that for every 10,000 women on drospirenone pills, there would be about 10 cases of blood clots over a year. That compared with six per 10,000 among women on older versions of the Pill.

Earlier this year, the FDA said it would add information on the possible risk to the products' labels.

According to ACOG, women should be aware of that information. But the choice of whether to use drospirenone pills, or any other form of birth control, should be up to women and their doctors, the group says.

Nanda said it's also important to remember that pregnancy carries a greater risk of blood clots than any birth control pill does.

For every 10,000 pregnant women, between five and 20 will develop a blood clot. And the risk goes up in the few weeks after delivery - to between 40 and 65 cases per 10,000 women.

Nanda advised discussing all your birth control options with your provider.

"Oral contraceptives are but one method in a mix that includes several, including long-acting IUDs and implants," Nanda said.

In fact, IUDs (or intrauterine devices) and contraceptive implants are considered the most effective forms of reversible birth control. That's because the devices are put in place by your doctor and last for several years; they don't rely on a couple's ability to use them "perfectly," like birth control pills and condoms do.

As for blood clot risks, some women are at greater risk than others and may need to avoid any type of birth control pill. Smokers who are age 35 or older are among them.

SOURCE: bit.ly/RDhkoW Obstetrics & Gynecology, November 2012.

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