Risk of Breast Cancer Higher Among Women With Current or Recent Hormonal Birth Control Use

A study of nearly 1.8 million women found that those currently or recently using hormonal birth control were at a higher risk of breast cancer than women who had never used hormonal birth control. Breast cancer risk increased with longer birth control use. The study was published in the New England Journal of Medicine.

Background

About 140 million women worldwide use hormonal birth control methods, whether as a pill, hormonal intrauterine device (IUD), injection, implant, patch or vaginal ring. These methods contain the hormones estrogen, progestin, or a combination of both.

Estrogen is known to drive the growth of certain types of breast cancer. The role of progestin remains uncertain. Many past studies on the relationship between hormonal birth control and breast cancer risk have not assessed the impact of the hormone types and the doses that are currently in use.

With so many women using these birth control methods, it’s important to understand the potential risk of developing breast cancer. This study focused specifically on women of age to become pregnant.

Design

The researchers used data collected by the Danish Sex Hormone Register Study, which tracks all women in Denmark between the ages of 15 and 79 from 1995 to present day. It includes medical information from seven nationwide Danish registries, including the Danish Cancer Registry, which tracks cancer diagnoses, and the National Register of Medical Product Statistics, which showed if and when a person filled a prescription for birth control.

For this study, the researchers looked at the records of 1,797,932 (about 1.8 million) women. The women that qualified for this study:

• were between 15 and 49 years of age between January 1, 1995 and December 31, 2012
• had no history of cancer
• had no history of blood clots that started in a vein, called venous thromboembolism
• had never had fertility treatment

The research team categorized the women’s records by whether their birth control use was

• current
• recent, meaning they stopped using birth control within 6 months of the start of the study
• previous, meaning they stopped using birth control more than 6 months before the start of the study

The women were followed for an average of 10.9 years, or until the first diagnosis of cancer or blood clot, death, leaving Denmark to live elsewhere, age 50, the start of fertility treatment, or the end of the follow-up period on December 31, 2012. The records were updated throughout the study to reflect changes in prescription or type of birth control used. If a woman became pregnant during the study, her record was held during the pregnancy and for 6 months after giving birth.

Results

During the study period, 11,517 breast cancer cases occurred in the women being followed. Compared with women who had never used hormonal birth control, the women who were current or recent users were 20 percent more likely to be diagnosed with breast cancer during the follow-up period.

Because the risk of breast cancer is low in young women, that means about one extra breast cancer diagnosis for every 7,690 women using hormonal birth control for one year.

Among women who used any method of hormonal birth control, risk of breast cancer increased significantly with longer use:

• Women who used it for less than one year were only 9 percent more likely than non-users to be diagnosed.
• Women who used it for 10 or more years were 38 percent more likely than non-users.

Longer use was also associated with increased risk after stopping the hormones:

• In women who used hormonal birth control for at least 5 years, risk remained higher for at least 5 years after stopping.
• Women who used hormonal birth control for fewer than 5 years only were at increased risk while they were using it.

The study found that breast cancer risk was similar across all hormonal birth control types. Women taking oral birth control pills faced similar breast cancer risk as those using IUDs, injections, implants and patches.

Limitations

This study lacked some information on other factors that impact breast cancer risk, including the age each woman had her first period, breastfeeding, alcohol use, and physical activity. Additionally, body mass index was only available for women who had given birth. The researchers also did not have record of whether participants had used hormonal birth control at any time before the start of the study.

What This Means For You

There are many reasons women use hormonal birth control, and learning that it increases the risk of breast cancer may be alarming. This study showed that women who use hormonal birth control are slightly more likely to be diagnosed with breast cancer. While this calculates to only one extra breast cancer diagnosis for every 7,690 women using hormonal birth control for one year, the actual risk might be greater for you if you have other risk factors for breast cancer such as a family history.  Talk to your healthcare provider about the pros and cons of different birth control methods and reassess hormone use with age.

Mørch, LS, Skovlund, CW, Hannaford, PC, et al. Contemporary Hormonal Contraception and the Risk of Breast Cancer. New England Journal of Medicine. December 7, 2017; 377:2228-2239. DOI: 10.1056/NEJMoa1700732.

This article was supported by the Grant or Cooperative Agreement Number 1 U58 DP005403, funded by the Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention or the Department of Health and Human Services.