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Weight and Birth Control Pill Effectiveness

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Updated May 28, 2014

Nearly half of all unintended pregnancies occur in women who report contraceptive use during the month they conceive. Research has shown a relationship between birth control pills and weight -- that a woman’s weight could contribute to oral contraceptive failure. Unplanned pregnancy and obesity represent overlapping epidemics in the United States. Women should understand that obesity and weight might decrease birth control pill effectiveness. Although birth control pills tend to be one of the most popular methods in preventing pregnancy, their effectiveness may be compromised in heavier women.

Current Status

Obesity rates have increased over the past 25 years. In fact, according to the National Center for Health Statistics, obesity continues to be a public health concern in the United States and throughout the world. In 2005 to 2006, more than one-third of U.S. adults (over 72 million people) were classified as being obese. This study also revealed that 35.3% of women suffer from obesity. Along the same notion, about 34% of the U.S. adult population (27.4% of females) would be considered overweight. Obesity is defined as a body mass index (BMI) of 30 or greater whereas an overweight person has a BMI of 25 to 29.9. BMI is calculated from a person’s weight and height and provides a reasonable indicator of body fatness and weight categories that may lead to health problems.

Background

Prior to the Holt et al. study, it was believed that body weight had no effect upon birth control pill effectiveness. This conclusion was primarily based on the Oxford Family Planning Association cohort study published in 2001. These researchers found no association between body weight and oral contraceptive failure rates (after adjusting for age and parity). However, 75% of the women in this study were using birth control pills that contained greater than or equal to 50 mcg of estrogen. The results from this study may not be applicable to current day oral contraceptive use because (with the exception of a handful of pill brands), the majority of combination birth control pills contain 30 to 35 mcg of estrogen, and several low-estrogen (20 mcg) varieties are also available.

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