Low estrogen birth control pills reduce clot risk

Some oral contraceptives are linked to a higher risk of blood clots than others, two new studies suggest.

It is well known that taking birth control pills containing both estrogen and progestogen slightly increases the risk of blood clots in legs and lungs. The latest European research shows the risk varies depending on the dose and type of the hormones.

The safest birth control pills were those with the lowest amounts of estrogen (20 micrograms of ethinylestradiol), and the levonorgestrel or norgestrel forms of progestogen, the Dutch and Danish research teams concluded.

Both studies also found the risk decreases the longer a woman takes the combined pill (with highest risk of a clotting problem in the first three months of use), and that progestogen-only pills and intrauterine devices that release hormones were not associated with an increased risk.

The overall likelihood that a woman will develop a blood clot when taking the Pill is low.

According to figures from the World Heath Organization, of 100,000 women using oral contraceptives for one year, about 15 to 25 will have a deep vein thrombosis, meaning the type of pill increases the risk of clots that are quite rare. The risk of clots increases much more during pregnancy than when taking oral contraceptives.

"Women should consult with their doctors and choose from all options on the Canadian market to get the safest method of contraception that will also afford the non-contraceptive benefits that they desire," said Robert Reid, chair of the division of reproductive endocrinology and infertility at Queen's University in Kingston, Ont.

Reid noted the risk of five blood clots per 10,000 women on the Pill is much less than risk of a blood clot in pregnancy, which can reach 100 to 200 per 10,000 pregnancies around the time of delivery.

Frits Rosendaal of the department of clinical epidemiology at Leiden University Medical Centre in the Netherlands and his colleagues found that different birth controls increased the relative risk, based on their analysis of 1,524 premenopausal women under the age of 50 who had deep vein thromboembolisms and 1,750 controls without clotting problems:

• Contraceptive pills containing levonorgestrel, such as Alesse, increased the risk of clots by 3.6-fold compared with women not taking birth control.
• Pills that contain gestoden, such as Femodette, increase the risk 5.6-fold compared with non-users.
• Pills with cyproterone acetate, such as Diane, had a 6.8-fold increase in risk versus non-users.
• Oral contraceptives made with drospirenone, such as Yasmin, showed a 6.3-fold increase in risk versus non-users.

The second study, led by Ojvind Lidegaard of the Rigshospitalet gynecological clinic at Copenhagen University, found similar results among Danish women aged 15 to 49 with no history of cardiovascular disease or cancer.

Low risk varieties 'sensible'

Between 1995 and 2005, 4,213 venous thrombotic events were observed, including 2,045 among women who were taking oral contraceptives. The increased risk based on higher levels of estrogen and type of progestogen were about the same as those found in the Dutch study.

While the Dutch study also asked participants about a family history of thrombotic disease, it is still not clear whether genetic predisposition makes a difference, Dr. Nick Dunn of the University of Southampton medical school said in a journal editorial accompanying the study.

"Patients with a personal or family history of venous thromboembolism should not take combined oral contraceptives," Dunn wrote. "Contrary to popular belief, smoking is not a risk factor for venous thromboembolism, and obesity (despite being associated with venous thromboembolism) is not a contraindication to using the pill, although choosing low risk varieties is sensible."

This risk is thought to decrease with time since some women have a genetic tendency to blood clots that only reveals itself when she is exposed to hormones in higher levels, such as taking birth control or when pregnant, Reid said.

Doctors prescribe birth control pills based on what's best for each woman, taking into consideration her risk of side-effects, family history, and her desire to relieve acne or symptoms of acne or pre-menstrual syndrome.

Women who use birth-control pills also have a lower risk of developing endometrial cancer and ovarian cancer.