Obesity and Contraception Effectiveness

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Birth control failure happens for a number of reasons, like inconsistent adherence to pill schedules and condom mishaps. Fortunately, it appears that weight is not one of those reasons.

For years, researchers have speculated about how obesity might interfere with the effectiveness of hormonal birth control. While research is still ongoing, the current opinion of the American College of Obstetricians and Gynecologists is that most forms of hormonal birth control, at the standard dose, are equally effective in "normal-weight" and obese women.

Body Mass Index (BMI)

Obesity rates have increased over the past two decades and continue to be a public health concern in the United States and throughout the world. In 2017-2018, more than 42% of people in the U.S. were considered obese.

Obesity is defined as a body mass index (BMI) of 30 or greater. BMI is calculated from a person’s weight and height and provides a reasonable overview of their body type. Weight categories like these help doctors identify people at risk of weight-related health problems.

Weight Status Categories
BMI Weight Status
Below 18.5 Underweight
18.5 - 24.9 Normal (healthy) weight
25.0 - 29.9 Overweight
30.0 and above Obese 

BMI and Birth Control

When you gain weight, it isn't only your waistline that changes. There is a lot more going on under the surface. The physical consequences of obesity are varied and extensive. Everything from the joints, to the heart, to the brain can be affected.

Those extra pounds also affect the way your body gets things done, like digesting your food and pumping your blood. Sometimes, this also means that people who are obese metabolize drugs differently than other people.

Over the last decade, as obesity rates continued trending upward, researchers starting asking questions about the way hormonal birth control methods worked in obese women. They worried that differences in the ways drugs are metabolized (processed) in the body might be reducing the efficacy of birth control pills, IUDs, implants (Nexplanon), and injections.

After a multi-year effort to involve more overweight and obese women in clinical trials, it seems the researchers' fears were mostly unfounded.

What the Science Says

In the past, research on obesity and hormonal contraception has been limited and of poor quality. Studies typically didn't include enough overweight and obese women to give a full picture of how birth control works in this special population. Many small studies ended up producing contradictory results.

Contradictory Research

  • Some past studies found slightly higher rates of unintended pregnancy among obese women on combined hormonal birth control pills, patches, and injections.
  • Other past studies found slightly lower rates among obese women, compared to normal-weight women.

Later, better-quality research has emerged to provide a clearer picture of the connection between birth control and body weight. A 2020 review of birth control studies factoring in body weight concluded that obesity does not influence the efficacy of most hormonal birth control methods.

The researchers determined that while there may be some minor differences in drug metabolism between normal-weight and obese women, there is no difference in the end result of pregnancy prevention.

However, researchers did find that Plan B (levonorgestrel), also known as emergency contraception or the morning-after pill, was not quite as effective in women with obesity.

It's important to note that contraceptive patches (brand names Twirla and Xulane) are not approved for use in women with a BMI over 30.

Overall, birth control failures are most often due to missed pills or failure to use birth control methods on schedule—not obesity.

Birth Control Options

Overweight and obese women have many of the same birth control options as everyone else. If you want the best, foolproof pregnancy prevention, you should consider long-term or permanent options. These include:

  • IUDs: These low-maintenance devices are 99% effective and last up to 12 years.
  • Nexplanon: The implant is another low-maintenance option that is 99% effective and lasts up to five years.
  • Injection: These are 94% effective when taken on schedule, every three months.
  • Surgical sterilization: Getting your tubes tied is a permanent procedure that is 99% effective.

Other effective birth control options include:

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Article Sources
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  1. ACOG practice bulletin no. 206: Use of hormonal contraception in women with coexisting medical conditions. Obstetrics & Gynecology. 2019;133(2):e128-e150.

    doi:10.1097/AOG.0000000000003072 

  2. Centers for Disease Control and Prevention. Adult obesity facts. Reviewed February 11, 2021.

  3. Ramanadhan S, Jusko WJ, Edelman A. Pharmacokinetics of hormonal contraception in individuals with obesity: a review. Curr Obstet Gynecol Rep. 2020;9(2):72-78. doi:10.1007/s13669-020-00284-y

  4. Dragoman MV, Simmons KB, Paulen ME, Curtis KM. Combined hormonal contraceptive (CHC) use among obese women and contraceptive effectiveness: a systematic review. Contraception. 2017;95(2):117-129. doi:10.1016/j.contraception.2016.10.010

  5. U.S. National Library of Medicine. DailyMed. Twirla prescribing information. Updated September 11, 2020.

  6. U.S. National Library of Medicine. DailyMed. Xulane prescribing information. Updated February 4, 2021.

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