Obesity and Contraception Effectiveness

The Link Between Weight and Birth Control Failure

Obesity and Contraception Effectiveness. Photo © 2014 Dawn Stacey

The most critical factor in preventing unplanned pregnancies is the use of safe and effective contraception by women who do not want to conceive. Yet nearly half of all unintended pregnancies occur in women who report using contraception during the month they that conceived.

One of the issues that may be contributing to birth control failure is a woman’s weight. Obese women may not realize that their weight could be compromising the effectiveness of their contraceptive method.

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. 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.

Unfortunately, contraceptive research has not included obese and overweight women in clinical trials. This has resulted in a limited body of data regarding contraceptive effectiveness and safety in obese and overweight women. That's unfortunate, given that effective contraception is even more important for obese women given the fact their higher risk of pregnancy-related complications.

To gain a greater understanding of how contraceptive effectiveness can be reduced by obesity, I researched an article on UpToDate -- a trusted electronic reference used by many doctors and patients:

"Many metabolic processes are affected by adiposity, and these changes in metabolism could potentially affect contraceptive efficacy. Since changes in metabolism are more contingent upon adiposity than on body proportions (ie, body mass index [BMI]), weight is probably more relevant than BMI in determining variability in contraceptive efficacy. Higher weight correlates with an increased metabolic rate. In particular, the clearance of hepatically metabolized drugs, such as contraceptive steroids, increases with increasing body weight. Theoretically, the half life of these drugs might be shorter in obese women and serum levels might be insufficient to maintain a contraceptive effect. In addition, obese women have a larger circulating blood volume than normal weight women. This may cause significant dilution of the concentration of contraceptive steroids, thereby decreasing contraceptive efficacy. Furthermore, contraceptive steroids are absorbed by adipose tissue, so women with more adipose may have less steroid available to circulate.

"Based on the potential effects of obesity on the pharmacokinetics of contraceptive steroids, it is tempting to simply recommend doubling the dose of contraceptive used by obese women. However, side effects and risks would also increase. As an example, a high dose of estrogen containing contraceptives would increase the risk of deep venous thrombosis and would be compounded by the risk already present for obese women."

So, what does this mean? Let's break down this important information.

Metabolic processes are biological processes that a living cell or organism uses to provide energy necessary for life and growth. Metabolism refers to the chemical reactions in the body's cells that convert the fuel from food into the energy our bodies need to function. People with higher weights (because of too much body fat) appear to have higher metabolic rates. Hepatically metabolized drugs, such as hormonal contraceptives, are absorbed by the digestive system and reach the liver before the rest of the body. The liver then metabolizes the drug, so that the concentration of the drug is greatly reduced by the time it enters the circulatory system. Overweight women may also be more likely to have higher levels of enzymes in the liver that will break down the hormones faster. So, since there is more tissue through which blood must circulate, the levels of circulating hormones may be decreased. This amount may not contain concentrations high enough to provide pregnancy protection (i.e., prevent ovulation, thicken cervical mucus, or thin the uterine lining).

Also, the half-life of a drug (basically, how long it takes for half of it to be eliminated from the bloodstream) may be shorter for overweight women because it is metabolized quicker -– so, there may not be enough contraceptive hormone left in the body to have a contraceptive effect.

Another factor has to do with blood volume circulation. In order to be effective, contraceptive hormones 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, due to higher amounts of blood circulating around the bodies. Due to the relatively low hormone levels in contraceptives, the larger blood volume could thin out the hormones and make them less effective.

Complicating the situation, estrogen and progestin are stored in adipose tissue. The more fat cells a woman has, the greater the chances that the hormones will become trapped in the fat instead of flowing through the bloodstream.

Because of the way an overweight woman’s body may absorb, distribute, metabolize and eliminate hormonal contraceptives, there are those who argue that contraceptive effectiveness can be maintained if the hormonal amount in the contraceptive is doubled. This poses a problem, however, due to potential side effects. Including higher doses of estrogen in contraceptives could increase the risk for developing side effects such as deep venous thrombosis (blood clots), stroke, high blood pressure or heart attacks -- risks already associated with obesity.

So when it comes to weight and birth control effectiveness, it is always important to compare the risks of unintended pregnancy against the advantages and disadvantages associated with a particular contraceptive method. In general, methods, where contraceptive failure is more likely to occur in obese women than in women of normal weight, include: oral contraceptives, the contraceptive patch, and Implanon/Nexplanon.

For obese women, IUDs and surgical sterilization may be more effective contraceptive methods. However, because of the increased body mass, these procedures may be harder to complete.

Other methods not affected by weight are hysteroscopic sterilization (Essure) and barrier methods such as condoms, female condoms, the sponge, diaphragms and cervical caps.

Want to learn more? See UpToDate's topic, "Contraception counseling for obese women," for additional in-depth medical information on counseling obese women about their contraceptive options and exploring how weight can contribute to contraceptive failure.

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  • Edelman, Alison and Kaneshiro, Bliss. "Contraception counseling for obese women." UptoDate. Accessed: May 2011