NEW YORK (Reuters Health) - Women who weigh more than most other women their same age may be more likely to accidentally become pregnant while they are taking oral contraceptive pills, according to a report released Tuesday.

Women who weighed 150 pounds or more were 1.6 times more likely to experience an oral contraceptive failure as their peers who weighed less, the investigators found.

They also note that the dose of the oral contraceptive played a role in the failure rate. Women who weighed the most and were taking "very-low dose" oral contraception were 4.5 times more likely to get pregnant compared to woman who weighed less on a similar dose. Women on a "low-dose" were 2.6 times more likely to get pregnant, the report indicates.

Over the last 20 years there has been a trickle of evidence that has linked a woman's weight with oral contraceptive failure, the authors note. The current study, conducted by Dr. Victoria L. Holt of The University of Washington in Seattle and colleagues, is published in the May issue of the journal Obstetrics and Gynecology.

When used correctly, oral contraceptives are a highly effective means of preventing pregnancy. Typically, about 3% of oral contraceptive users will become pregnant in a year of use.

In their investigation, Holt and her team interviewed 618 women aged 18 to 39 who had originally taken part as a "control" group in a study of ovarian cysts conducted in the early 1990s.

The researchers went back and asked if they had become pregnant and what type and dose of oral contraception they had been using. The women also told the researchers their height and body weight at the time of their pregnancy. Overall, 106 pregnancies occurred, for a rate of about 3.8% in a year of use. The researchers divided the women into four groups by weight, and found those in the top 25% were at higher risk of accidental pregnancy.

Holt and colleagues point out that they had no information about the consistency of birth control pill use among the women in their study and "one could hypothesize that less conscientious contraceptive use by heavy women might account for their increased likelihood of oral contraceptive failure."

On the other hand, they note that a comparison between type of contraception use and body weight indicated that heavy women were not more likely to get pregnant using "barrier" methods like condoms or diaphragms. This would suggest, according to the researchers, that method failure rather than non-compliance may be to blame with oral contraceptives.

The heavier women were also more likely to be smokers and were more likely to be African American compared with women in the other groups, the report indicates.

Holt's team adds that larger studies need to replicate their findings and that, ultimately, "consideration of a woman's weight may be an important element of oral contraception prescription."

It is not clear why extra pounds might cause contraception to fail but it is possible that heavier women's "overall enhanced metabolic rate leads to more rapid drug metabolism and subsequent insufficient serum progestin or estrogen levels for good contraceptive efficacy," Holt and colleagues conclude.

SOURCE: Obstetrics and Gynecology 2002;99:820-827.

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