title: Body weight and risk of oral contraceptive failure.
abstract: OBJECTIVE
  To examine the hypothesis that higher body weight increases the risk of oral contraceptive (OC) failure.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, Washington, USA. vholt@u.washington.edu
authors.forename: Victoria L
authors.initials: VL
authors.lastname: Holt
authors.forename: Kara L
authors.initials: KL
authors.lastname: Cushing-Haugen
authors.forename: Janet R
authors.initials: JR
authors.lastname: Daling
mesh_terms: D000042:Absorption; D000273:Adipose Tissue / Q000378:metabolism; D000293:Adolescent; D000328:Adult; D015992:Body Mass Index; D001835:Body Weight; D015331:Cohort Studies; D003267:Contraception; D003276:Contraceptives, Oral / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D007407:Interviews as Topic; D011247:Pregnancy; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D012307:Risk Factors; D011795:Surveys and Questionnaires
chemical_list: D003276:Contraceptives, Oral