Breaking News Emails

Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.
 / Updated 
By Maggie Fox

Newer birth control pills raise the risk of a blood clot by as much as or more than older formulations, researchers report — but the risk is still very low.

The study found that women taking some of the newer formulations had about four times the risk of a blood clot , called venous thromboembolism or VTE, as a woman not taking any birth control pill.

But the overall risk is still very low — just 14 cases out of every 10,000 women — the researchers report in the BMJ, the online journal of the British Medical Association.

That’s far lower than the risk of being pregnant. Pregnant women have 10 times the risk of a blood clot. That’s because estrogen — contained in most birth control pills — makes the blood more likely to clot, says Dr. Barbara Levy, vice president for health policy for the American College of Obstetricians and Gynecologists.

“In layman’s terms, it makes the blood a little thicker,” Levy told NBC News. “Pregnancy is a high estrogen state that really increases the risk of blood clots.”

“Pregnancy is a high estrogen state that really increases the risk of blood clots.”

Doctors have known for years that some of the newer contraceptives carry a higher risk of blood clots than older ones do. It’s not clear why, but the newer formulations use a different type of a second hormone, called progesterone or progestin.

“Different progestins have different effects,” said Levy, who was not involved in the study.

Yana Vinogradova of Britain’s University of Nottingham and colleagues looked at data from 10,000 women across Britain. About half of them had been 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,” Vinogradova’s team wrote. Those are the newer progestins, found in brands such as Yaz, Yasmin and Marvelon.

“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,” they added. That’s the form found in older pills such as Microgynon.

Levy says she is not sure how solid the results are. “The way that this study was done was not perfect and the conclusion may or may not be correct,” she cautioned. “The biggest risk for blood clots is in the first six months that you take pills,” she added. And women just starting out on the Pill are far more likely to get one of the newer formulations, she said.

“The way that this study was done was not perfect and the conclusion may or may not be correct."

So the effect may not be because of the pills themselves, but because the women were new to the Pill and at higher risk anyway.

Either way, Levy says, birth control pills are very safe for most women. “People shouldn’t go off their pills because they are scared to death of blood clots,” she said.

Blood clots can be treated with anticoagulant drugs.

ACOG says doctors should consider an individual woman’s risk of blood clots before prescribing any formulation of birth control pill.

Risk factors include smoking, being over 35, having major surgery that keeps a woman in bed for a long time, lupus and inflammatory bowel disease.

Levy says what's most important is that women find a form of contraception that suits them. Some women get oily skin, acne, weight gain or depression from some formulations. And gynecologists note that intrauterine devices— IUDs —can be far safer and more effective than other forms of contraception.