U.S. health regulators are adding information to the labels on a popular class of birth control pills that includes Bayer's Yaz and Yasmin, to show they may raise the risk of blood clots.

All common birth control pills increase a woman's chances of getting potentially fatal blood clots.

But the U.S. Food and Drug Administration said on Tuesday that some studies showed the danger may be even higher for more recent pills that contain the compound drospirenone, a synthetic hormone.

"The revised drug labels will report that some epidemiologic studies reported as high as a three-fold increase in the risk of blood clots for drospirenone-containing products ... whereas other epidemiological studies found no additional risk," the FDA said in a statement.

The FDA's own study found that 10 in 10,000 women taking pills with drospirenone would get a blood clot per year, compared with about six in 10,000 women taking older contraceptives.

A clot in blood vessels can prove fatal if it breaks loose and travels to the lungs, heart or brain.

To put the risk into perspective, the FDA added that the risk of blood clots from pregnancy is even higher than any risk from birth control pills.

The announcement comes after an advisory committee of outside experts to the FDA voted in December for a label revision for pills with drospirenone, calling for clearer information about their risks and benefits.

The experts stopped short of agreeing that these pills' risks outweighed their benefits, since some studies found the pills did not increase blood clot risks.

In January, the Project on Government Oversight, a U.S. watchdog group, urged the FDA to hold a new vote about the birth control pills after a study showed four of the FDA's advisers had ties in the past to makers of pills containing drospirenone.

The FDA said past ties to industry are usually not considered conflicts under the law.

During the December panel meeting, some women's advocacy groups called for the pills to be taken off the market, as the studies that viewed pills favorably were industry-funded.

The consumer groups and patients shared tearful stories about sudden deaths or life-changing disabilities they or their loved ones suffered from blood clots, which they believed were caused by Yaz or Yasmin.

Yaz, a reformulated version of Yasmin, remains one of the U.S. top-selling contraceptives. Bayer had $374 million in Yaz sales in 2010, according to data from IMS Health. Sales of Yaz have slipped in recent years, after Teva Pharmaceutical Industries and Watson Pharmaceuticals launched generic versions.