Heaviest women have the most pill failures. (FYI).

A woman's weight may affect how well oral contraceptives work for her, according to results of a retrospective cohort analysis of 755 randomly selected members of a health maintenance organization in Washington State. (1) In the course of nearly 3,000 person-years of pill use, 106 pregnancies (or 3.8 per 100 person-years of use) occurred. The pregnancy rate was 5.6 per 100 person-years of use among women in the highest quartile of body weight, compared with 2.7-3.6 per 100 among those in the three lower quartiles. Analyses controlling for parity confirmed that women in the highest quartile had a significantly greater risk of experiencing pill failure than did all other women combined (relative risk, 1.6). The difference was not significant among users of high-dose pills, but the risk was elevated among those taking oral contraceptives with less than 50 mcg and, particularly, less than 35 mcg of estrogen (2.6 and 4.5, respectively). While acknowledging that they lacked detailed data about women's weight and contraceptive use that would lead to more definitive conclusions, the researchers remark that a woman's weight may "affect metabolism sufficiently to compromise contraceptive effectiveness" and may be an important consideration in her choice of the pill.

(1.) Holt VL, Cushing-Haugen KL and Daling JR, Body weight and risk of oral contraceptive failure, Obstetrics & Gynecology, 2002, 99(5, pt. 1):820-827.