Using the Pill

Weight and Birth Control Pill Effectiveness

October 9, 2018

Almost half of all unintended pregnancies occur in women who report contraceptive use during the month they conceive. Studies have shown a connection between birth control pills and weight — a female’s weight could contribute to oral contraceptive failure. Unplanned pregnancy and obesity represent overlapping epidemics in the United States. Women should realize that obesity and weight might decrease birth control pill efficacy.

Though birth control pills are normally one of the most popular techniques of preventing pregnancy, their efficacy may be jeopardized in heavier women.

Current Status

Obesity rates have increased over the previous 25 years. In fact, according to the National Center for Health Statistics, obesity has been a public health issue in the USA and throughout the world. In 2005 to 2006, more than one third of U.S. adults (over 72 million individuals ) were classified as being overweight. This study also demonstrated that 35.3% of women suffer with obesity. Along the exact same notion, about 34% of the U.S. adult inhabitants (27.4% of females) would be considered overweight. Obesity is defined as a body mass index (BMI) of 30 or higher whereas an overweight person has a BMI of 25 to 29.9. BMI is calculated from a individual’s weight and height and offers a reasonable indicator of body fatness and weight categories that may result in health problems.


Prior to the Holt et al. research, it was considered that the body fat had no effect on birth control pill effectiveness. This conclusion was mainly based on the Oxford Family Planning Association cohort study published in 2001. These investigators found no connection 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 outcomes from this study may not be applicable to current day oral contraceptive usage because (with the exception of a handful of pill manufacturers ), nearly all combination birth control pills contain 30 to 35 mcg of estrogen, and several low-estrogen (20 mcg) forms are also offered.

Recent research

Holt et al. conducted the biggest case-control study so far, analyzing the connection between weight and oral contraceptive failure. They concluded that for women using birth control pills (compared to women of lower weight), those who are overweight are 60% more likely to get pregnant while those that are obese are 70 percent more likely to experience contraception failure. Particularly, the link between extra pounds and tablet failure surfaced among obese women whose BMI was 27.3 or greater (this would be equivalent to some 5-foot, 4-inch woman who weighs 160 pounds or more). Thus, women who always use oral contraceptives and whose BMI was higher than 27.3’d 1.58 times the possibility of becoming pregnant compared to constant users whose BMI was less than 27.3.

Also, an overweight woman is more likely to experience contraception failure when she misses her everyday pill. It’s important to notice, however, variables such as weight, height, adherence to a birth control schedule, and frequency of sexual intercourse had been self-reported in this study. This usually means that result inaccuracies might be possible due to faulty coverage.

A 2007 research study by Brunner, Huber, and Toth reveals a weak, but not statistically significant, relationship between obesity and birth control pill failure. The results did indicate that overweight women (BMI ≥ 30) had a higher risk for pregnancy.

However, after the researchers adjusted for its era, race/ethnicity, and parity of the girls, they concluded that there was no association between weight and oral contraceptive failure. The researchers also did advise their research could have yielded faulty consequences because instead of simply weighing and measuring the research participates, the results were based on the women’s self-report of the height and weight. Given that women tend to over-report their stature and under-report their weight by a few pounds, the BMI may have been incorrect. In the end, the researchers did not possess information regarding the frequency of sexual intercourse or whether the women were consistently taking their tablets; the lack of inclusion of these factors may significantly bias the results of the study, along with the investigators even concluded that bigger, more comprehensive studies are needed to have a more definite answer as to whether obesity plays relevant role in oral contraceptive efficacy.

Why the Pill Is Not as Effective

Unfortunately, the precise reason as to why overweight and/or obese women are at higher risk of oral contraceptive failure isn’t totally known. But, several suggested 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 very first introduced years ago).  Oral contraceptive manufacturers have decreased hormone levels in an effort to minimize undesirable side effects, for example danger of blood clots, weight reduction, and headaches. In order to be effective, the hormones in the pill need to circulate through a woman’s bloodstream. If a girl has a larger body mass, it might be more difficult for sufficient circulation to occur, particularly given lower level of hormones found in almost all of the pills.
  • Strength: Typically, heavier women have a higher metabolism, so the tablets hormones might be metabolized faster. So, the more a woman weighs, the greater her basal metabolic rate; this may shorten the length of the pill’s effectiveness.
  • Liver Enzymes: Overweight women may experience an up-regulation of liver metabolism; those high levels of enzymes might help divide the hormones in oral contraceptives. Heavier women tend to get a higher circulating blood circulation and body mass. Given that there is more tissue through which blood must circulate along with the likelihood that the enzymes will break the hormones down quicker, the amounts of circulating hormones may be decreased.
  • Hormone Storage: The hormones, estrogen and progestin, found in birth control pills are stored in your body fat. Therefore the more fat cells a woman has, the higher her odds of the pill hormones becoming trapped in the fat instead of flowing through her blood.

What Does This All Mean?

Should we translate the study to mean that overweight women should avoid the usage of birth control pills? This might not necessarily be the answer. In fact, the power of the use or oral contraceptives (even in severely overweight women) would nevertheless remain rather high. One of 100 women taking oral contraceptives for one year, Holt et al.’s (2005) study suggests 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 quantity of obesity-related complications of pregnancy, which can consist of gestational diabetes, high blood pressure, and Cesarean delivery.

Where It Stands

Many health care providers are deciding to counteract the decrease in tablets effectiveness by placing overweight and obese girls on a marginally higher-dose birth control pill as opposed to a low-dose new to help ensure there are sufficient hormones to prevent ovulation.

If you end up in this situation, it is important to explore all of your choices and risk factors with your doctor. Since overweight women may be more likely to have cardiovascular disease risk factors than normal-weight ladies, the higher-dose of oral contraception may increase these cardiovascular risks even more. By way of instance, studies have shown there to be an increased risk of venous thromboembolism (blood clots) among obese women who use birth control pills. Thus, a doctor may want to maintain an obese woman on a regular-dose birth control pill with the directions to use a backup method of birth control to help optimize pregnancy protection. In this case, obstruction methods such as female or male condoms, the sponge, or spermicide may be used along with the tablet computer. Finally, if an overweight woman has decided that she no longer wishes to have any more kids, a permanent form of contraception such as a tubal ligation or hysteroscopic (no-surgical) sterilization, such as Essure.

The Most Important Thing

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