Study looks at weight impact on OC efficacy

Research findings have suggested higher failure rates in oral contraceptive (OC) users who are heavy; however, new results from a retrospective analysis of Phase III efficacy data of a popular ethinyl estradiol/norgestimate formulation (Ortho Tri-Cyclen Lo, Ortho McNeil Pharmaceutical; Raritan, NJ) indicate that it is not associated with an increased risk of pregnancy in such women.1

Two earlier studies, a retrospective cohort study followed by a more rigorous case-control analysis, indicated that overweight and obese women who take oral contraceptives may be 60%-70% more likely to get pregnant while on the Pill than women of lower weight.2,3

The new data on Ortho Tri-Cyclen Lo is of interest to family planning providers. The pill was the leading choice for young nonsmoking women in the 2005 and 2004 CTU Contraception Surveys.

The current study provides reassuring information that the weight of the patient will not interfere with the contraceptive efficacy of Ortho Tri-Cyclen Lo, says Susan Wysocki, RNC, NP, president and chief executive officer of the National Association of Nurse Practitioners in Women's Health. However, more research is needed on other low-dose formulations when it comes to the issue of weight and OC efficacy, she adds.

In the current study, investigators performed a retrospective analysis of Ortho Tri-Cyclen Lo's Phase III clinical data to determine the association of pregnancies with body weight and body mass index (BMI). Data came from 1,673 pill users, weighing from 90 to 240 pounds, for 11,003 use cycles. The 13-cycle cumulative probabilities of pregnancy were calculated by life-table analysis using a Kaplan-Meier method to estimate the probability of no pregnancies through 13 cycles of pill use.

The researchers found that a total of 20 pregnancies occurred. Fourteen were likely due to method failure and six due to user failure (such as forgetting to take pills). This number translated into a total pregnancy probability of 1.9%. The failure rate of the Pill in perfect use was 1.5%. [In Contraceptive Technology, the failure rate in all women (typical use) is 8%, and the perfect use failure rate is 0.3%.4] The probability of total and method-failure pregnancies was the same across different body weights. In the lowest weight decile (86 to 113.5 pounds), two pregnancies were recorded among 168 women. In the highest weight decile (175 to 240 pounds), there were four pregnancies among the 174 women. However, the difference was not statistically significant.

Women in the study who weighed 155 pounds and above (n = 435) did not experience significantly higher pregnancy risk than those weighing less than 155 pounds with either method-failure pregnancies (relative risk 1.03, p = 0.95) or total pregnancies (relative risk 1.42, p = 0.46). The results were the same when other weight criteria and BMI criteria were applied.

What are the strengths of the study in showing efficacy of the drug in heavier women?

The current analysis is based on data from a prospective clinical trial in which recall bias and many confounding factors do not exist, explains Katherine LaGuardia, MD, director of medical affairs at Ortho Women's Health & Urology. All of the analyses from various weight/BMI categories and different methods indicate no statistically significant association of higher pregnancy risk among heavier women, she points out.

"Although the original study was not designed for this analysis, our sample is comparable to the previous observational studies which drew a conclusion on the association between weight and low-dose oral contraceptive efficacy," LaGuardia states.

References

  1. Zhang HF, LaGuardia KD, Creanga DL. Higher body weight and BMI are not associated with reduced efficacy in Ortho Tri-Cyclen Lo users. Presented at the Annual Clinical Meeting of the American College of Obstetricians and Gynecologists. Washington, DC; May 2006.
  2. Holt VL, Scholes D, Wicklund KG, et al. Body mass index, weight, and oral contraceptive failure risk. Obstet Gynecol 2005; 105:46-52.
  3. Holt VL, Cushing-Haugen KL, Daling JR. Body weight and risk of oral contraceptive failure. Obstet Gynecol 2002; 99:820-827.
  4. Hatcher RA, Trussell J, Stewart F, et al. Contraceptive Technology: 18th revised edition. New York City: Ardent Media; 2004.