Birth control pills seem to be effective on all women, irrespective of their physical makeup, weight and size, a new study has revealed to challenge existing belief that contraceptives consumed orally may not prevent pregnancy among obese women.

As part of the study, which had 226 women volunteers aged between 18 and 35 in the normal to skinny to overweight category, the researchers assigned them to take either a low or high dose version of a birth control pill.

After three or four months of using regular birth control pills, the women were checked to see if it was suppressing ovulation. The research team found that out of the 150 women who consumed the pills regularly, ovulation occurred in three of the 96 normal-weight women and in one of 54 obese women.

As for the women who failed to take the pills regularly, they seemed likely to ovulate more than the other two groups, according to the results of the study published in the August issue of the medical journal Obstetrics & Gynecology.

"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," says 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.

In addition, the study also revealed that there was no difference in ovulation among the women who took lower-dose contraceptives to those who were given pills of the higher-dose type, especially in the case of obese women.

Doctors said this was an important discovery since earlier it was thought that obese women required the higher-dose version to curtail ovulation. It was suggested that these women may be at a higher risk of developing blood clots from taking higher-dose pills.

The knowledge that both higher dose and lower dose pills will work with obese women and protect them from health risks will make physicians treat such patients differently in the future, the researchers noted.