The association between body weight, unintended pregnancy resulting in a livebirth, and contraception at the time of conception

Annually, 3 million pregnancies in the United States are classified as unintended, with many of these unintended pregnancies occurring to women who use some type of contraceptive. Obesity may be affecting the biological effectiveness of contraceptives. We investigated whether there is an association between body weight and unintended pregnancy and whether this association differs by a woman's contraceptive status at the time of conception. We conducted a case-control study using multistate data from the 1999 Pregnancy Risk Assessment Monitoring System. A total of 18,445 women provided complete information on pregnancy intention, contraceptive use at the time of conception, weight, height, and other covariates. Multivariable logistic regression was used to obtain odds ratios (ORs) and 95% confidence intervals (CIs). Among contraceptors, overweight and obese women had nearly twice the odds of having an unintended pregnancy as compared to women with normal body mass indices (OR=1.73 [95% CI: 1.20, 2.36] and OR=1.75 [95% CI: 1.21, 2.52], respectively) after adjustment for age, marital status, education, socioeconomic status, Medicaid, race/ethnicity, and parity. No association was found between heavier weight and unintended pregnancy among noncontraceptors. Though the observed associations could be the result of selection bias or unmeasured confounding, the findings suggest that the effect of obesity on unintended pregnancy is limited to contraceptive failure. Prospective studies designed specifically to examine a body weight-contraceptive failure association are needed to determine if heavier women should be advised to use contraceptive methods other than hormonal contraceptives to prevent unintended pregnancy.