Weight and Birth Control Pill Effectiveness

Woman and birth control pills on couch

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Nearly half of all unintended pregnancies occur in women who report contraceptive use during the month they conceive. Research has shown a relationship between birth control pills and weight—that a woman’s weight could contribute to oral contraceptive failure. Unplanned pregnancy and obesity represent overlapping epidemics in the United States. Women should understand that obesity and weight might decrease birth control pill effectiveness. Although birth control pills tend to be one of the most popular methods of preventing pregnancy, their effectiveness may be compromised in heavier women.

Current Status

Obesity rates have increased over the past 25 years. In fact, according to the National Center for Health Statistics, obesity continues to be a public health concern in the United States and throughout the world.

In 2016, more than one-third of U.S. adults (over 70 million people) were classified as being obese. This study also revealed that 41.1% of women are obese. Along with the same notion, about 34% of the U.S. adult population (27.4% of females) would be considered overweight.

Obesity is defined as a body mass index (BMI) of 30 or greater whereas an overweight person has a BMI of 25 to 29.9.

BMI is calculated from a person’s weight and height and provides a reasonable indicator of body fatness and weight categories that may lead to health problems.


Prior to the Holt et al. study, it was believed that bodyweight had no effect on birth control pill effectiveness. This conclusion was primarily based on the Oxford Family Planning Association cohort study published in 2001. These researchers found no association between body weight and oral contraceptive failure rates (after adjusting for age and parity). However, 75% of the women in this study were using birth control pills that contained greater than or equal to 50 mcg of estrogen. The results from this study may not be applicable to current day oral contraceptive use because (with the exception of a handful of pill brands), the majority of combination birth control pills contain 30 to 35 mcg of estrogen, and several low-estrogen (20 mcg) varieties are also available.


Holt et al. conducted the largest case-control study thus far, examining the link between weight and oral contraceptive failure. They concluded that for women who use birth control pills (as compared to women of lower weight), those who are overweight are 60% more likely to get pregnant while those who are obese are 70% more likely to experience contraception failure.

Specifically, the connection between extra pounds and pill failure first surfaced among overweight women whose BMI was 27.3 or higher (this would be equivalent to a 5-foot, 4-inch woman who weighs 160 pounds or more). Thus, women who consistently use oral contraceptives and whose BMI was greater than 27.3 had 1.58 times the risk of becoming pregnant as compared with consistent users whose BMI was less than 27.3. Also, an overweight woman is more likely to experience contraception failure if she misses her daily pill.

It is important to note, however, factors including height, weight, adherence to a birth control schedule, and frequency of sexual intercourse was self-reported in this study. This means that result inaccuracies may be possible due to faulty reporting.

A 2007 research study by Brunner, Huber, and Toth reveals a weak, although not statistically significant, relationship between obesity and birth control pill failure. The results did indicate that obese women (BMI ≥ 30) had a higher risk of pregnancy. Yet, after the researchers adjusted for the age, race/ethnicity, and parity of the women, they concluded that there was no association between weight and oral contraceptive failure.

The researchers did advise that their study could have yielded faulty results because rather than weighing and measuring the research participates, the results were based on the women’s self-report of their height and weight. Given that women tend to over-report their height and under-report their weight by a few pounds, the BMI may have been inaccurate.

Finally, the researchers did not have information on the frequency of sexual intercourse or whether or not the women were consistently taking their pills; the lack of inclusion of these factors could significantly bias the results of this study, and the researchers even concluded that larger, more comprehensive studies are needed to have a more definite answer as to whether obesity plays relevant role in oral contraceptive effectiveness.

Why the Pill Is Less Effective

Unfortunately, the exact reason as to why overweight and/or obese women are at greater risk of oral contraceptive failure is not completely known. However, several proposed theories point to biological factors that may account for increased risk:

  • Hormone levels: Modern-day birth control pills contain relatively low hormone levels (as compared to those first introduced decades ago). Oral contraceptive manufacturers have decreased hormone levels in an attempt to minimize unwanted side effects, such as the risk of blood clots, weight gain, and headaches. In order to be effective, the hormones in the pill need to circulate through a woman's bloodstream. If a woman has a larger body mass, it could be more difficult for adequate circulation to happen, especially given a lower level of hormones found in most of today’s pills.
  • Metabolism: Typically, heavier women have a higher metabolism, so the pill hormones may be metabolized faster. So, the more a woman weighs, the higher her basal metabolic rate; this can shorten the duration of the pill’s effectiveness.
  • Liver enzymes: Overweight women may experience an up-regulation of liver metabolism; these higher levels of enzymes may help break down the hormones in oral contraceptives. Heavier women tend to have a greater circulating blood volume and body mass. Given that there is more tissue through which blood must circulate and the likelihood that the enzymes will break down the hormones faster, the levels of circulating hormones may be decreased.
  • Hormone storage: The hormones, estrogen, and progestin, found in birth control pills are stored in body fat. Thus, the more fat cells a woman has, the greater her chances of the pill hormones becoming trapped in the fat instead of flowing through her bloodstream.

What Does This All Mean?

Should we interpret the research to mean that obese women should avoid the use of birth control pills? This may not necessarily be the answer. In fact, the effectiveness of the use of oral contraceptives (even in severely overweight women) would still remain fairly high.

Among 100 women taking oral contraceptives for one year, Holt et al.’s (2005) study suggests that an additional two to four women will get pregnant due to being overweight or obese. However, this increased risk of pregnancy could also equate to a higher number of obesity-related complications of pregnancy, which can include gestational diabetes, high blood pressure, and Cesarean delivery.

Where It Stands

Many health care providers are choosing to counteract the decrease in pill effectiveness by putting overweight and obese women on a slightly higher dose birth control pill rather than a low-dose brand to help ensure that there are enough hormones to block ovulation.

If you find yourself in this situation, it is important to discuss all of your options and risk factors with your doctor. Since overweight women may be more likely to have cardiovascular disease risk factors than normal-weight women, the higher-dose of oral contraception could increase these cardiovascular risks even more. For example, research has shown there to be an increased risk of venous thromboembolism (blood clots) among obese women who use birth control pills.

Therefore, a doctor may wish to keep an overweight woman on a regular-dose birth control pill with the instructions to use a backup method of birth control to help maximize pregnancy protection. In this case, barrier methods like male or female condomsthe sponge, or spermicide may be used in conjunction with the pill.

Finally, if an overweight woman has decided that she no longer wishes to have any more children, a permanent form of contraception such as a tubal ligation or hysteroscopic (no-surgical) sterilization, like Essure.

The Bottom Line

Given that there is a slight connection between higher weight and birth control pill effectiveness, it is important to discuss this with your healthcare provider. Also, since first being prescribed the pill, if you notice that your weight has noticeably gone up (maybe, perhaps at least two dress sizes), make sure to inform your healthcare provider to make sure that this method is still the most effective and safest contraceptive option for you.

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