THURSDAY, Aug. 19 (HealthDay News) -- Birth control pills are equally effective in obese and thinner women, claims a study that challenges the belief that oral contraceptives may not reliably prevent pregnancy in heavier women.

The study included 226 women, aged 18 to 35, who were normal weight or overweight. They were randomly assigned to take either a lower- or higher-dose version of a birth control pill.

After three or four months of using the pill (the time it takes for a woman's body to get used to the contraceptive), the women were checked to see if the pill was suppressing ovulation.

Of the 150 women who used the pill consistently, ovulation occurred in three of the 96 normal-weight women and in one of the 54 obese women. Women who were not consistent about taking the pill were more likely to ovulate, said the researchers at the New York-Presbyterian Hospital/Columbia University Medical Center.

"Our findings strengthen the message to patients that the pill will only work if it is taken every day. Weight does not seem to have an impact on suppression of ovulation, but consistency of pill-taking does," principal investigator Dr. Carolyn Westhoff, a professor of obstetrics and gynecology and director of the family planning division at Columbia University College of Physicians and Surgeons, said in a medical center news release.

The researchers also found that the lower-dose oral contraceptive was as effective as the higher-dose type in suppressing ovulation in obese women. This is an important finding because it was previously thought that obese women might need the higher-dose version. However, obese women are at increased risk for developing blood clots from taking birth control pills, and higher-dose pills increase that risk.

"Knowing that the lower dose works as well as the higher dose will allow physicians to not only help women with obesity avoid unwanted pregnancies, but also protect them from the possible health risks associated with higher doses," said Westhoff, who is also an obstetrician/gynecologist at New York-Presbyterian Hospital/Columbia University Medical Center.

"For a woman to fear relying on her oral contraceptive to prevent pregnancy is a huge burden. This study should put those fears to rest," Westhoff said.

The study appears in the August issue of Obstetrics & Gynecology.

More information

The American College of Obstetricians and Gynecologists has more about birth control.