Elsevier

Annals of Epidemiology

Volume 15, Issue 7, August 2005, Pages 492-499
Annals of Epidemiology

The Role of Body Weight in Oral Contraceptive Failure: Results from the 1995 National Survey of Family Growth

https://doi.org/10.1016/j.annepidem.2004.10.009Get rights and content

Purpose

Many unintended pregnancies occur in women who use contraception. We conducted this study to determine if increasing body weight is associated with oral contraceptive (OC) failure.

Methods

This retrospective cohort study consists of the 1916 women who reported using OCs in January 1993 and provided complete covariate information on the 1993 National Health Interview Survey and 1995 National Survey of Family Growth. Body weight and body mass index (BMI) were self-reported in 1993. The outcome was defined to be any conception occurring in women reporting OC use during the month of conception. Cox proportional hazards models were used to model the body weight/BMI–OC failure association.

Results

Women with a BMI ≥ 30 had a statistically significant increased risk of having an OC failure as compared to women with BMIs of 20 to 24.9 (HR = 1.80, 95% CI, 1.01, 3.20). However, after adjustment for age, marital status, education, poverty, race/ethnicity, parity, and dual method use, this increased risk was attenuated and no longer statistically significant (HR = 1.51, 95% CI, 0.81, 2.82). Increasing body weight was not associated with an increased risk of OC failure in the unadjusted or adjusted models.

Conclusions

We did not find a strong or statistically significant association between increasing body weight/BMI and OC failure among this population of women. Prospective studies specifically designed to examine this association are needed to determine if heavier women should be advised to use a contraceptive method other than OCs to prevent pregnancy.

Introduction

Each year, 3 million of the 6 million pregnancies in the US are classified as unintended (1). It is estimated that nearly half of these unintended pregnancies occur in the 90% of women who use some type of contraceptive (2). Researchers attribute these contraceptive failures to noncompliance and ineffective use 2, 3, 4, 5. Few studies have investigated whether biologic factors, rather than ineffective use, may be responsible for the large number of pregnancies that occur in women using contraceptives. Body weight is one such biologic factor that may affect how contraceptives work, specifically hormonal contraceptives. Excess weight may cause an enhanced metabolic rate and hence more rapid drug metabolism (6). Suggestions of the association between higher body weight and increased contraceptive failure emerge from secondary analyses of efficacy trials of Norplant and the transdermal contraceptive patch 7, 8, 9, 10. In 2002 Holt et al. (6) also demonstrated an association between higher body weight and increased risk of oral contraceptive (OC) failure in an analysis whose primary purpose was to examine the association between body weight and risk of OC failure. We used data from the 1995 National Survey of Family Growth (NSFG), a large survey of US women, to further investigate if increasing body weight is associated with OC failure in a retrospective cohort study.

Section snippets

Study Population and Design

The 1995 NSFG sample was drawn from respondents of the 1993 National Health Interview Survey (NHIS), a survey designed to provide information on the health of the civilian, noninstitutionalized, household population of the US. Through personal interviews with a national sample of women 15 to 44 years of age who responded to the 1993 NHIS, the NSFG aimed to collect more detailed data on factors affecting pregnancy and women's health. In 1995 trained personnel conducted interviews with 10,847

Results

The majority of study subjects were under 30 years of age, married, white, and well educated (Table 1). Hispanic and less educated women were at increased risk of OC failure (Table 2). Additionally, women with BMIs ≥ 30 had a statistically significant increased risk of having an OC failure as compared to women in the 20 to 24.9 BMI category (HR = 1.80, 95% CI, 1.01, 3.20). Results were similar when performed on the subpopulation of fecund and subfecund women (data not shown).

The Cox proportional

Discussion

In this study, we did not find a statistically significant association between increasing body weight or BMI and OC failure after adjustment for age, marital status, education, poverty level, race/ethnicity, parity, dual method use, and fecundity status.

The present study has several limitations. Nondifferential misclassification of the exposure is possible as weight was self-reported by the study participants. Though the validity of self-reported weight has been questioned, a number of studies

References (43)

  • M.J. Rosenberg et al.

    Use and misuse of oral contraceptives: Risk indicators for poor pill taking and discontinuation

    Contraception

    (1995)
  • S.K. Henshaw

    Unintended pregnancy in the United States

    Fam Plann Perspect

    (1998)
  • N. Ranjit et al.

    Contraceptive failure in the first two years of use: Differences across socioeconomic subgroups

    Fam Plann Perspect

    (2001)
  • H. Fu et al.

    Contraceptive failure rates: New estimates from the 1995 National Survey of Family Growth

    Fam Plann Perspect

    (1999)
  • P.J.A. Hillard

    Oral contraception noncompliance: The extent of the problem

    Adv Contracept

    (1992)
  • M.C. Audet et al.

    Evaluation of contraceptive efficacy and cycle control of a transdermal contraceptive patch vs. an oral contraceptive: A randomized controlled trial

    JAMA

    (2001)
  • M. Zieman et al.

    Contraceptive efficacy and cycle control with the Ortho Evra/Evra transdermal system: The analysis of pooled data

    Fertil Steril

    (2002)
  • B.M. Zaadstra et al.

    Fat and female fecundity: Prospective study of effect of body fat distribution on conception rates

    BMJ

    (1993)
  • T.K. Jensen et al.

    Fecundability in relation to body mass and menstrual cycle patterns

    Epidemiology

    (1999)
  • F. Bolumar et al.

    Body mass index and delayed conception: A European multicenter study on infertility and subfecundity

    Am J Epidemiol

    (2000)
  • J.W. Rich-Edwards et al.

    Physical activity, body mass index, and ovulatory disorder infertility

    Epidemiology

    (2002)
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