Clot Risk Varies Among Birth Control Pills

Women may not be using the safest brands of birth control pills when it comes to the risk of developing blood clots.

Two new studies in the British Medical Journal show levels of risk differing with each brand of pill, based on the types of hormones and doses of estrogen in the pills.

More than 11 million women use oral contraceptives in the United States; birth control pills are the most popular form.

The studies also quantified the known link between taking birth control pills and an increased risk of potentially fatal blood clots, finding a five-fold increase in blood clots among women who take the pill, that's six women in 10,000 a year who will have a serious clot.

While the risk of having getting a blood clot from taking the pill is extremely low, CBS News medical correspondent and OB/GYN Dr. Jennifer Ashton said on "The Early Show" Friday, it's helpful for women and their doctors to know which types may have the lowest risk.

Ashton observed there are dozens of birth control pills with varying amounts of hormones, from brand name to generic options. The study found that a type of hormone called Levonorgestrel and a low dose of estrogen is the safest for blood clots. Women, Ashton said, need to go over this carefully with their doctor and find the lowest dose pill that is safe and effective for them.

"Women, by and large, don't really know there's a difference," she said.

Most combination pills contain an estrogen and a progesterone, Ashton explained. Women, she said, should ask for the lowest amount of estrogen, asking for 20 micrograms of estrogen. She added women should ask for a progesterone that begins with the letter "L" or "N" because they are the safest.

Ashton said, "The majority of women, they think (birth control pills are) all the same, and they don't ask the doctor about the differences."

And though the pill is generally considered safe, some women shouldn't take the pill. Ashton said women who are older than 35 and smoke, have a family history of a clotting disorder or are over 35 who get classic migraines shouldn't take oral contraceptives.

However, the pill does have some benefits for women, Ashton said. Women who take the pill, she said, have a lowered risk for uterine and ovarian cancer. But, she reminded viewers, birth control pills do not protect against STDs, so practice safe sex.