The possibility that contraceptives are more likely to fail in obese patients is only one reason why you need to take a different tack when counseling them about their most appropriate birth control options. . .
. . . You need to advise them, too, that combination oral contraceptives (COCs) may put overweight or obese women at increased risk for both arterial and venous thromboembolic disease-as may certain other birth control methods. These concerns, combined with the ever-growing obesity epidemic among reproductive-aged American women, make it imperative for us to focus family planning efforts on our overweight patients.
Obese women are at higher risk of unintended pregnancy than their non-obese counterparts. Data from a study of more than 24,000 women, which examined the intendedness of pregnancy, contraceptive use, body weight, and BMI at the time of conception, suggested that overweight and obese women using contraception were more likely than non-obese women to unintentionally get pregnant.2 This difference wasn't seen in women who weren't using birth control at the time they conceived. These findings suggest that the overall effectiveness of contraceptives is decreased in overweight and obese women.