Obesity and Oral Contraceptive Efficacy
Obesity and Oral Contraceptive Efficacy
Abstract & Commentary
By Alison Edelman, MD, MPH, Assistant Professor, Assistant Director of the Family Planning Fellowship, Department of Obstetrics & Gynecology, Oregon Health & Science University, Portland, is Associate Editor for OB/GYN Clinical Alert.
Dr. Edelman is a consultant to Schering-Plough and receives grant/research support from the Society for Family Planning.
Synopsis: Oral contraceptives with a less than 30 mcg ethinyl estradiol component may be less effective in obese women.
Source: Burkman RT, et al. Association between efficacy and body weight or body mass index for two low-dose oral contraceptives. Contraception 2009;79:424-427.
A large prospective randomized trial (n = 2812 women) was retrospectively analyzed to look at contraceptive efficacy and weight/BMI. This study compared oral contraceptives containing 25 µg ethinyl estradiol/180/215/250 norgestimate vs 20 µg ethinyl estradiol/1 mg norethindrone. Weight and BMI were dichotomized to either < or ≥ 70 kg or < or ≥ 25 mg/kg2. The median baseline weight was 62.5 kg (range, 39.9-108.9 kg) and median BMI was 23 kg/m2 (range, 15.9-47.6 kg/m2). In total, 37 pregnancies occurred. A slight but not statistically significant increase in the relative risk (RR) of pregnancy was found in the ≥ 70 kg and ≥ 25 kg/m2 groups (RR, 1.25; confidence interval [CI], 0.63-2.46; and RR, 1.85; CI, 0.98-3.45, respectively).
Commentary
Concern regarding how obesity impacts contraceptive efficacy has been a focus of recent debates, but there is a significant paucity of data upon which to base any clinical recommendations. Prior contraceptive efficacy trials have excluded women greater than 130% of ideal body weight and recent cohort studies have been conflicted in their findings — some have demonstrated an adverse effect of weight on contraceptive efficacy and others have not.1-5 Burkman and colleagues have retrospectively analyzed data from their randomized clinical trial of 2 different oral contraceptives. They found a small effect of obesity on oral contraceptive efficacy but this finding was not statistically significant. This is one of the only studies to have prospectively followed pregnancy rates in obese women using oral contraceptives.
However, there are several significant limitations to this study. Although these data were obtained from a randomized trial, the outcome (how weight/BMI affects efficacy) was not originally planned; thus, the analysis may contain unknown bias. In addition, this study excluded women of > 32 kg/m2. A BMI of 30 kg/m2 is the starting definition of obesity,6 so in actuality this study mainly included a population of overweight and not obese women (even though it does appear that they had a protocol breach and enrolled some women over their set BMI cut off of 32 kg/m2).
So unfortunately, we still don't have a good answer to our question — does obesity affect oral contraceptive efficacy? Nevertheless, should we move to change our contraceptive prescribing practices in obese women based on some of these concerns? Currently, there is not enough information to recommend avoiding the use of oral contraceptives in obese women. But there is enough information even in normal BMI women that birth control methods like the intrauterine device and the implant are more effective than oral contraceptive pills and we should be promoting these to women of any weight.7
References
- Holt V, et al. Body weight and risk of oral contraceptive failure. Obstet Gynecol 2002;99(5 Pt 1):820-827.
- Holt V, et al. Body mass index, weight, and oral contraceptive failure risk. Obstet Gynecol 2005;105:46-52.
- Brunner Huber LR, et al. Body mass index and risk for oral contraceptive failure: A case-cohort study in South Carolina. Ann Epidemiol 2006;16:637-643.
- Westhoff C, et al. Subject weight and oral contraceptive efficacy in recent clinical trials. Contraception 2008;78:167.
- Vessey M. Oral contraceptive failures and body weight: Findings in a large cohort study. J Fam Plann Reprod Health Care 2001;27:90-91.
- World Health Organization. BMI categories. Available at: www.euro.who.int/nutrition. Accessed Oct. 7, 2008.
- Kost K, et al. Estimates of contraceptive failure from the 2002 National Survey of Family Growth. Contraception 2008;77:10-21.
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