Can Weight Affect Birth Control Effectiveness?

It had long been suggested that being overweight or obese might undermine the effectiveness of birth control pills and other hormonal contraceptives, leading to unplanned pregnancy. It is true that excess body weight can pose health concerns to women for many reasons, but more recent research suggests that decreased efficacy of hormonal birth control may not be one of them.

Obesity and Fertility

Body mass index (BMI) is calculated from a person’s weight and height and provides a reasonable indicator of obesity and its associated health risks.

By definition, obesity is a body mass index (BMI) of 30 or greater, whereas an overweight person has a BMI of between 25 and 29.9.

Among them, women with obesity are known to be three times more likely to experience infertility due to hormonal disruptions that impair menstruation and ovulation.

Given this, it seems backwards that birth control pills designed to prevent pregnancy are assumed to be less effective in the very women who are more likely to experience infertility.

Adding to the confusion, scientists have yet to establish how extra fat is meant to alter the way that the pill is handled by the body.

Research has long been conflicted on these issues, and only recently have scientists begun to test the hypothesis in a more qualitative way.

Evolution of Research

Much of the confusion surrounding the efficacy of birth control pills in obese women stemmed from earlier research that compared BMI values to pregnancy rates. On their face, findings were often quite compelling.

For instance, a 2010 review in the journal Obstetrics & Gynecology, which involved 39,531 women and 11 clinical trials, concluded that heavier women were statistically at a higher risk of unplanned pregnancy while on the pill than women of healthy weight.

However, a closer look at the studies revealed inconsistencies in some of the findings. Among them, some studies suggested that higher weight, not higher BMI, was associated with contraceptive failure. Others found no such link between weight and unplanned pregnancy.

In fact, of the 11 studies reviewed, only four concluded that BMI was linked to a higher risk of contraceptive failure in overweight or obese women.

The review went further to suggest that injectable or implantable contraceptives may be affected by body mass even though the evidence of this was low with injectables and there were no pregnancies reported in the implant studies. Conclusions were instead based on blood hormone concentrations.

Differences in study populations, methods, materials, measures, and controls made it difficult to draw any consistent conclusions other than to say that weight may play a role in contraceptive failure rates. Or it may not.

Study Shows No Loss of Effect

To better understand how weight may or may not affect birth control pills, researchers from New York Presbyterian Hospital and Columbia University Medical Center designed a randomized controlled study in which 226 women between the ages of 18 and 35 were assigned either a low- or high-dose version of the pill. Half of the women were of healthy weight and the other half were obese.

Of the 150 women who took the pill consistently, three of the 96 healthy-weight women ovulated (meaning that the treatment failed). By comparison, only one of 54 women with obesity experienced such failure. The failure rates were statistically comparable, meaning that obesity played no part in them.

The one factor that did influence contraceptive efficacy was treatment adherence. According to the investigators, the failures were linked to inconsistent dosing rather than BMI or any other weight or metabolic factor. Other studies have supported these findings, albeit with limitations.

A 2016 review published in the Cochrane Database of Systematic Reviews (involving 17 studies and 63,813 women) concluded that there was no association between BMI or weight and the effectiveness of hormonal contraceptives.

That is not to suggest, however, that all hormonal contraceptives are equally effective in heavier women. There is, in fact, evidence that certain combination birth control pills may underperform in obese or overweight women and lead to an unplanned pregnancy.

Combination Pill Concerns

Combination birth control pills, comprised of different forms of estrogen and progestin, are generally safe and effective in women with obesity. These include pills containing the synthetic estrogen ethinyl estradiol and progestin medications like drospirenone, levonorgestrel, norethindrone acetate, or norgestimate.

Of the available combinations, there is evidence that birth control pills containing ethinyl estradiol and norethindrone acetate may be twice as likely to fail in women with a BMI over 25 compared to women of normal weight.

The reasons for this are not entirely clear, although some experts suggest that obesity may alter the clearance or half-life of norethindrone, slowing the rate at which the drug reaches therapeutic levels in the blood.

With that said, there is no evidence of this occurring when norethindrone is taken on its own (in progestin-only "mini-pills"), leaving the theory largely proven.

Where there is agreement is that research into the effectiveness of birth control pills in women with obesity is sorely lacking. Rarely are heavier women included in pill efficacy studies and, if they are, their weight is hardly ever factored into the results.

Because of this, manufacturers of Lo Loestrin (ethinyl estradiol and norethindrone tablets) and Generess (ethinyl estradiol and norethindrone chewable) have decided to step out in front of the controversy and warn consumers that their drugs' "safety and efficacy in women with BMI over 30 kg/m2 has not been evaluated."

Alternate Dosing Strategies

There are currently no guidelines on the use of birth control pills in overweight or obese women. There are, however, some experts who endorse the continuous use of low-dose birth control pills; i.e., the pill is taken every day (rather than in cycles) to completely suppress menstruation.

Others suggest that high-dose birth control pills taken cyclically, as is typical, should be used in place of low-dose contraceptives if overweight.

In both cases, there is evidence that the strategies offered higher sustained blood hormone levels in women with BMIs over 30. What is lacking, however, is any proof that taking continuous or high-dose birth control pills if you are obese will result in any better or worse outcomes compared to women of healthy weight.

Even the researchers contend that the main factor affecting contraceptive failure rates was not weight or medication dosages but poor treatment adherence. As such, the benefits of a dose adjustment may be more presumed than real insofar as weight is concerned.

And there are also risks to consider. For example, the use of birth control pills in obese women is linked to an increased risk of venous thromboembolism (blood clots in the deep veins of the leg, groin, or arm). It is unknown if higher doses might increase that risk.

It is also yet to be established how safe continuous oral contraceptives are over the long term.

Bariatric Surgery Caution

Weight loss is not a means of contraception. However, it may theoretically improve the effectiveness of oral contraceptives. To this end, some women with obesity have turned to bariatric surgery as a means to achieve rapid, "safe" weight loss.

While the surgery offers benefits to women with BMIs over 40, it is more likely than not to reduce, rather than improve, the effectiveness of oral contraceptives.

With some procedures, like the Rouex-Y gastric bypass, the reduced size of the stomach reduces the absorption of the pill. It is for this reason that the U.S. Medical Eligibility Criteria for Contraceptive Use (USMEC) advises against the use of oral contraceptives in women undergoing the procedure.

Other Birth Control Options

If you are overweight or obese and are concerned about the effectiveness of birth control pills in preventing pregnancy, there are other options you can explore, some of which have proven effective in women with higher BMIs.

In place of oral contraceptives, you may want to consider these hormonal options:

Non-hormonal barrier birth control options include:

Permanent means of birth control include:

Talking With Your Doctor

If you are overweight and are worried about whether the pill will fail you, speak with your gynecologist.

They can walk you through other methods of birth control if there are reasonable medical concerns or you would simply feel more comfortable changing your method of contraception. Some options may be more appropriate for you than others.

With that said, the failure of the pill usually has more to do with poor adherence—missed doses, inconsistent dosing—that your weight or waist size. If you aren't taking your birth control pills as you should, ask your gynecologist for tips and advice on how to improve your adherence.

A Word From Verywell

According to the National Center for Health Statistics, more than one-third of adults in the United States are classified as obese. Of them, 44.7% are women between the ages of 20 and 39. In some racial and ethnic groups, including Hispanics and African Americans, women were more likely to be obese than men.

All this to say, if you are overweight or obese, you are not alone. While you may hear about quick-fix solutions, remember that fad diets rarely work over the long term and may cause more harm than good. If you intend to lose weight, do so safely with exercise and a healthy, balanced diet.

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