The risk of having a heart attack or stroke is very low for most women who take low-dose hormonal contraceptives, but that risk rises with age, new research confirms.

The study, published online in The New England Journal of Medicine, included more than 1.6 million Danish women. That makes it the largest study ever to examine the impact of different formulations of birth control pills and other forms of hormonal contraception on blood clots that can lead to heart attack and stroke.

Slideshow: Birth Control Options
Low-Dose Estrogen Pill Safer

Researchers also confirmed that new-generation, low-dose estrogen oral birth control is far safer than early versions of the pill.

"Risk today is significantly lower than it was decades ago in the era of high-dose pills," says University of Copenhagen professor Ojvind Lidegaard, MD, who led the research.

Hormonal contraceptives that did not contain estrogen, such as the progestin-only IUD sold in the U.S. as Mirena, were not found to raise the risk for blood clot-related heart attack and strokes at all.

But birth control delivered via skin patches and vaginal rings was associated with a significantly elevated blood clot risk, similar to that seen in older high-dose estrogen pills.
Two-Fold Increase in Risk

In related research published last fall, Lidegaard and colleagues linked use of new-generation birth control pills containing the progestin hormone drospirenone with a higher risk of blood clots in the legs that can dislodge to blood vessels in the lungs (also known as a pulmonary embolism).

The oral contraceptives Yaz, Yasmin, and Ocella contain drospirenone. In April, the FDA concluded that birth control pills containing drospirenone "may be associated with a higher risk for blood clots than other progestin-containing pills." The FDA will put that on the pills' labels.

But in the new study, these products were no more likely to cause blood clots that lead to heart attacks and stroke than any other combination contraceptive.

Age was the single-most important risk factor for developing blood clots related to hormonal contraceptive use.

The researchers followed 1.6 million Danish aged 15-49 from 1995 until 2009. During this time, there were about 3,300 clot-related strokes and 1,700 clot-related heart attacks among the women studied.

Use of combined oral contraceptives with ultra-low doses of estrogen (20 mcg) was associated with a 50% increase in risk, while use of traditional low-dose pills (30 to 40 mcg) was associated with an 80% increase in risk.

Overall, the study shows that heart attacks and strokes caused by blood clots were twice as likely in women using combination oral contraceptives. But the risk is still very low for most women, especially those younger than 35 with few risk factors for these events, Lidegaard tells WebMD.

"A 20-year-old with a very low baseline risk for heart attack and stroke should not be all that concerned that her risk may be double, but a woman who is 35 or older with 10 times the baseline risk may want to choose another form of birth control."