Body weight and risk of oral contraceptive failure

Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, Washington, USA.
Obstetrics and Gynecology (Impact Factor: 5.18). 06/2002; 99(5 Pt 1):820-7. DOI: 10.1016/S0029-7844(02)01939-7
Source: PubMed


To examine the hypothesis that higher body weight increases the risk of oral contraceptive (OC) failure.
We conducted a retrospective cohort analysis of data from 755 randomly selected female enrollees of Group Health Cooperative of Puget Sound who completed an in-person interview and dietary questionnaire between 1990 and 1994 as control subjects for a case-control study of ovarian cysts. Among the 618 women who were OC ever-users, we used Cox proportional hazards regression models to estimate the relative risk (RR) of pregnancy while using OCs associated with body weight quartile.
During 2822 person-years of OC use, 106 confirmed pregnancies occurred (3.8 per 100 person-years of exposure). After controlling for parity, women in the highest body weight quartile (70.5 kg or more) had a significantly increased risk of OC failure (RR 1.6, 95% confidence interval [CI] 1.1, 2.4) compared with women of lower weight. Higher elevations of risk associated with the highest weight quartile were seen among very low-dose OC users (RR 4.5, 95% CI 1.4, 14.4) and low-dose OC users (RR 2.6, 95% CI 1.2, 5.9), controlling for parity, race, religion, and menstrual cycle regularity.
Our findings suggest that body habitus may affect metabolism sufficiently to compromise contraceptive effectiveness. Consideration of a woman's weight may be an important element of OC prescription.

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    • "While there has been concern about decreased effectiveness of hormonal contraceptives among obese women, the evidence is limited and inconsistent [8] [9] [10] [11] [12] [13] [14] [15] [16]. Obesity by itself might impair the effectiveness of hormonal contraception [14,17–19]. "

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    • "Education was divided into quartiles (Ͻ12 years, 12 years, 13–15 years, Ͼ15 years) in preliminary analyses but was subsequently collapsed into two categories (Յ12 years, Ͼ12 years) because the first two and the last two quartiles were similar. Interest in the effect of weight on the efficacy of medical abortion stems from reports of lower efficacy in the contraceptive patch in women who weighed at least 90 kg [7] and of lower efficacy of very low dose and low dose but not higher dose oral contraceptives among women in the highest weight category [8]. All the explanatory variables are categorical, and one category of each variable (the reference category) must be omitted in the hazard regressions to avoid perfect colinearity ; which category is omitted is completely arbitrary. "
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