Does increased weight impact OC efficacy?
Results of new research suggest that being overweight may increase women’s risk of becoming pregnant while using oral contraceptives (OCs).1 How do these findings play into contraceptive counseling for clinicians?
The new findings are quite similar to those found in a 2002 pilot study, says Victoria Holt, PhD, MPH, a professor in the department of epidemiology at the University of Washington in Seattle who served as lead author for both studies. In the pilot study, investigators found that women in the highest weight quartile or highest body mass index (BMI) quartile had a significantly increased risk of OC failure.2
Results from the new study indicate that overweight and obese women who take oral contraceptives are 60%-70% more likely to get pregnant while on the birth control pill than women of lower weight.2 Among 100 women taking OCs for a year, the new study results suggest that an additional two to four women will get pregnant due to being overweight or obese.
However, more research is needed on the subject before clinicians change their prescribing habits, says Paul Norris, MD, assistant professor of obstetrics and gynecology at the University of Miami who performed similar research. His investigation suggested a positive correlation between contraceptive failure in combination OC users and increased BMI; of 514 combination OC users, pregnancies occurred in seven women. Five of those women had a BMI greater than 25; three of those five had a BMI greater than 30.3
"So far, the data is light; I don’t think it is enough to change prescribing habits," he states. "When I lecture at medical school or if I’m doing lectures for a group, I will mention it, but I will say more data needs to come out."
What is the difference between being overweight and obese? The Geneva-based World Health Organization breaks weight down into four categories, defined by BMI (the measure of body fat based on height and weight): underweight (18.5 or lower), normal (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or greater).
In conducting the new study, researchers compared weight and body mass index of 248 women who became pregnant while on OCs to an age-matched comparison group of 533 nonpregnant Pill users. Study participants were enrolled in the Group Health Cooperative, a Seattle health maintenance organization.
Researchers found the association between extra pounds and pill failure first surfaced among overweight women whose body mass index was 27.3 or higher, which is equivalent to a 5-foot, 4-inch woman who weighs 160 pounds or more. These women faced a 60% greater risk of getting pregnant while on OCs, while those considered obese, with a BMI of 32.2 or greater, faced a 70% greater risk.
What factors could be at play in decreased efficacy? Researchers say possible factors could include increased metabolism, which can shorten the duration of a medication’s effectiveness; an increase in liver enzymes, which may lead to a drop in circulating blood levels of a drug; and possible storage of active ingredients in body fat, diminishing the impact of the drug.
What’s your move?
Other research does not indicate that clinicians need change their prescribing habits when it comes to advising OC use in overweight women.4,5
According to information provided by the Washington, DC-based National Association of Nurse Practitioners in Women’s Health (NPWH), a woman’s ultimate contraceptive decision should be based on:
- effectiveness;
- incidence of side effects;
- convenience;
- desired duration of contraception;
- reversibility;
- noncontraceptive benefits;
- cost. (See resource for more information on the subject.)
"Will it change my prescribing habits? No, not at all," says Norris of the new research. "Oral contraceptive pills are still very effective in the obese patient; even though there is an increased failure rate, pills are still very effective."
References
1. Holt VL, Scholes D, Wicklund KG, et al. Body mass index, weight, and oral contraceptive failure risk. Obstet Gynecol 2005; 105:46-52.
2. Holt VL, Cushing-Haugen KL, Daling JR. Body weight and risk of oral contraceptive failure. Obstet Gynecol 2002; 99:820-827.
3. Norris PM, Kamat A, Estes C, et al. Contraceptive failure in overweight patients taking combination oral contraceptive pills. Presented at the Association of Reproductive Health Professional Annual Meeting. San Diego; September 2003.
4. Kaunitz AM. Three new formulations. Female Patient 2002; 27(suppl):25-29.
5. Vessey M, Painter R. Oral contraceptive failures and body weight: Findings in a large cohort study. J Fam Plann Reprod Health Care 2001; 27:90-91.
The Washington, DC-based National Association of Nurse Practitioners in Women’s Health offers a continuing education module, "Contraceptive Failure and Body Weight: Fact or Fiction." Go to the organization’s web site, www.npwh.org. Click on "Other Current CE Opportunities," then "Contraceptive Failure and Body Weight: Fact or Fiction."
Results of new research suggest that being overweight may increase womens risk of becoming pregnant while using oral contraceptives. How do these findings play into contraceptive counseling for clinicians?
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