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Contraception Corner

Contraceptive Efficacy and Body Weight

Miriam Zieman, MD; Anita Nelson, MD

A relationship between body weight and contraceptive efficacy has been suggested in some studies of hormonal contraceptives but was studied in most detail with the recent introduction of the transdermal contraceptive system. This column will review the relationship between body weight and contraceptive efficacy for various hormonal contraceptives. In some efficacy trials of hormonal methods of contraception, obesity was an exclusion criteria, ie, women above certain weights were not included in the studies and a relationship between efficacy and body weight could not be analyzed. The trials of the transdermal contraceptive system did not exclude patients on the basis of weight, and had enough information, albeit a relatively small number of heavier women, to analyze the relationship between weight and efficacy.

Body weight and hormonal methods

Oral Contraceptives

A recent retrospective cohort study by Holt et al, found a significantly increased risk of oral contraceptive (OC) failure in women in the highest body weight quartile of their analysis. In this study, this was 70.5 kg, with failures of 3.9 per 100 person-years OC use (relative risk [RR] 1.7 95% confidence interval [CI] 1.1-2.9) compared with women of lower weight. In addition, women in the heaviest quartile experienced the greatest failure rates, 6.8 per 100 woman-years use, when taking pills with less than 35 µg ethinyl estradiol. (RR 4.5, 95% CI 1.4-14.4).¹

In older reports regarding progestin-only OCs (POPs), there was also an association of higher failure rates in heavier women. The Oxford Family Planning Association reported the lowest failure rate (0.5 per 100 woman-years) of POPs in the lightest women (less than 112 lb), and the highest rate 1.3, for women above 155 lb.²

Progestin-only Implants: The Norplant System

Studies of Norplant contraceptive implants³ and progestin-only vaginal rings³ also found significantly increased pregnancy rates in women greater than or equal to 70 kg. Cumulative 7-year failure rates of Norplant in women weighing more than 70 kg was five times greater than women weighing less than 50 kg. However, Norplant remained very effective in the heavy group with pearl rate less than 1 per 100 woman-years.³

Combination Vaginal Rings: The NuvaRing

In a 1-year multicenter study of the combination vaginal ring (etonogestrel 120 µg/ethinyl estradiol 15 µg), 1,145 women were enrolled in an efficacy and tolerability study. Specific body weights were not mentioned as an exclusion criterion, nor were they mentioned in the discussion of contraceptive failures.⁴

Transdermal Contraceptives: Ortho-Evra

To review the data as relates to the patch (ethinyl estradiol/norelgestromin), use in 3,319 women for 22,155 cycles resulted in an overall pearl index (number of pregnancies per 100 women in first year of use) of 0.88 (0.44-1.33 95% CI) with the life table analysis overall failure rate of 0.8% (0.3%-1.3% 95%CI). When looking specifically at failures when the method was used exactly as prescribed, the pearl index was 0.7 (0.31-1.10) and the life table analysis 0.6% (0.2%-0.9%). There was no association of contraceptive failure with age or race, but there was a significant increase in failures with women of higher weights⁶ (Table 1).

Regarding this data, it is reassuring that failures remain below 1% for all body weight groups up to 90 kg. However, above this weight, data from a relatively small number of women, 83, had a significantly higher failure rate (approximately 6 per 100 woman-years). Despite this, the patch is not contraindicated in these women: this higher failure rate is listed as a “precaution” in the product labeling. To understand this, we need to put these numbers into context, ie, compare them to published perfect use and typical use failure rates of other acceptable methods of contraception.⁶ For example, first-year perfect/typical use failure rates of the diaphragm with spermicide was 6%/16%; cervical cap with spermicide was 9%/16% in nulliparous women and 26%/32% in multiparous women; and spermicide alone was 15%/29%.

Contraceptive failure and body weight: possible mechanisms

Reasons for increased failure rates in heavier women would include possible metabolic differences or differences in compliance. In the OC study mentioned above, Holt et al analyzed a subset of women using male condoms as their method and found no difference in failure rates between the heavier and lighter women in condom users suggesting compliance was no different between these populations.¹ We may not be able to generalize results of compliance with condoms to that with oral medication. Regarding metabolic differences in heavier women, an increased metabolic rate can lead to more rapid steroid metabolism with resulting serum steroid levels being below threshold for contraceptive efficacy. Since steroids are lipophilic, they can also be stored in fat resulting in decreased circulating levels. We can speculate whether increased endogenous estrogen in heavier women may affect progestin thickening of cervical mucus. Weight is preferable to body mass index as the index variable because weight is more strongly associated with metabolism.²

Clinical Relevance

While we have reviewed that there is a suggestion of decreased efficacy in relation to body weight for various methods of hormonal contraceptives, there are little data at this point to make strong conclusions. To summarize, regarding the patch, a threshold weight of 90 kg was found, above which a clinically significant increase in failure occurred. Regarding low-dose pill data, one retrospective study found an effect at weights above 70.5 kg, but because the highest quartile was not broken down further (and analyzed differently), the threshold might be different. There have been reports regarding progestin-only pills and Norplant having higher failures in heavier women; however, efficacy was still excellent. When counseling heavier women who are choosing a method, we need to encourage use of the most effective available methods. To our knowledge, there have been no reports of higher failures in the injectable; unfortunately, these can cause slightly more weight gain than some of the other methods (specifically undesirable in heavier women). Intrauterine devices similarly have not been linked to higher failures and, in the appropriate candidates, would be an excellent choice. We would not hesitate to give OCs, either combined or progestin-only,to heavier women based on the results of one retrospective study. One question raised is whether to give higher dose OCs to heavier women based on the Holt data. Although in Holt’s study, the 50 µg (and above) formulations were the most effective in heavier women, we would not prefer these because of the increased risk of venous thromboembolic events (VTE) with these pills, with obesity being another risk factor for VTE. One could choose a 35-µg pill over a lower dose without decreasing the safety margin and possibly increasing efficacy. Another approach would be using a barrier method as an added safeguard or perhaps continued use of a lower dose formulation, but shortening the pill-free interval (not based on evidence that this would increase efficacy). When we counsel a woman about her contraceptive choices, we present all options but recommend the most effective ones. We would not recommend the patch as a first line choice for a woman weighing more than 90 kg just as we do not routinely recommend spermicide alone as a method for any population since these are not among the most effective first line options. On the other hand, we would not withhold the patch in this population if this is the method particularly desirable, and the alternatives a woman would be willing to use were one of the less effective methods. Additional pharmacokinetic and prospective clinical studies need to be done to give us more complete data on this important subject as it relates to all the hormonal methods.

REFERENCES

1. Holt V L, Cushing-Haugen K, Daling JR. Body weight and risk of oral contraceptive failure. Obstet Gynecol. 2002;99:820-827.
2. McCann MF, Potter LS. Progestin-only contraception: A comprehensive review. Contraception. 1994;50 (6 Suppl 1):S1-S195.
3. Gu S, Sivin I, Du M, et al Effectiveness of Norplant implants through seven years: A large scale study in China. Contraception. 1995;52:99-103.
4. Roumen FJ, Apter D, Mulders TM, et al. Efficacy, tolerability and acceptability of a novel contraceptive vaginal ring releasing etonogestrel and ethinyl oestradiol. Hum Reprod. 2001;16:469-475.
5. Zieman M, Guillebaud J, Weisberg E et.al, Contraceptive efficacy and cycle control with the Ortho Evra/Evra transdermal system: the analysis of pooled data. Fertil Steril. 2002;77: S13-S18.
6. Hathcer RA, Trussell J, Stewart F, Cates W. Contraceptive Technology. 18th ed. New York, NY: Irvington Publishers; In press.

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ARHP is a nonprofit, national medical organization that has been educating front line providers and their patients since 1963. The organization and members are dedicated to educating physicians and health care providers, and the public about important reproductive health issues including contraception, sexually transmitted diseases, menopause, abortion, sexuality, and infertility. ARHP contact information: 2401 Pennsylvania Ave, N.W., Suite 350, Washington DC 20037-1718; ph: (202) 466-3825; fax: (202) 466-3826; e-mail: arhp@arhp.org; Web: www.arhp.org.

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