Current and recent use of hormonal contraceptives was associated with a 20 percent increased risk of breast cancer.
The overall increased risk was small.
To put things in perspective, the overall risk posed by taking the pill for one year amounts to one extra breast cancer case for every 7,690 women aged 15 to 49, the researchers calculated.
Dr Chris Zahn, ACOG's vice president for practice activities, acknowledged a link between breast cancer risk and hormone use, but urged concerned women to consult a trusted medical provider before making changes.
Mørch said that "knowledge is needed on the potential beneficial influence of newer contraceptives on the risk of ovarian and colorectal cancer, since evidence now relates to older types of hormonal contraceptives".
The non-oral progestin-only levonorgestrel-releasing intrauterine system also showed an adjusted relative risk increase for breast cancer (1.21, 95% CI 1.11-1.33). However, he noted that the clinical implications of this study "must be placed in the context of the low incidence rates of breast cancer among younger women", pointing out that most of the new breast cancer cases occurring in the study were among women using oral contraceptives over the age of 40. "But it does show an increased risk, so for people who don't have a great reason for taking oral contraceptives, or are amenable to alternatives, perhaps they should think about it".
The study, which followed 1.8 million Danish women for more than a decade, upends widely held assumptions about modern contraceptives for younger generations of women.
The paper did not make any note of whether birth control impacted mortality from breast cancer, Leath noted.
The study shows that "the search for an oral contraceptive that does not elevate the risk of breast cancer needs to continue", said Dr. David Hunter of the University of Oxford in a Journal editorial.
Officials with the American College of Obstetricians and Gynecologists said that they would carefully evaluate the new findings, but emphasized that hormonal contraceptives are for many women "among the most safe, effective and accessible options available".
Two types of birth control pills are sold in the US - one that combines synthetic versions of the hormones estrogen and progesterone, and the "minipill" that only delivers progestin, a synthetic formulation of progesterone.
"The relative risk increase in this study is only 1.2 on average".
"And there is also the reassuring thought that oral contraceptive use may decrease the risk of ovarian and endometrial cancer".
Today, most versions of the pill contain between 15 and 35 micrograms of estrogen, Gaudet said. Still, the additional risk would result in a comparatively few additional cases of breast cancer, the researchers said. For a 20-year-old woman, for example, the probability of developing breast cancer in the next 10 years is 0.06 per cent, or 1 in 1,732, according to breastcancer.org. "In particular the knowledge of risk with newer progestins was sparse".
Mørch explained to MedPage Today that "there was a lack of evidence on contemporary hormonal contraception and risk of breast cancer". That roughly translates to a 12 percent lifetime risk for a woman, although many factors affect breast cancer risk.
"No type of hormone contraceptive is risk-free unfortunately", said lead author Lina Morch of Copenhagen University Hospital.
Not only that, but there are plenty of non-hormonal birth control options to consider, like the non-hormonal IUD, diaphragm and condoms - and let's not forget vasectomies.