Weight and See
Study questions effectiveness of birth control pills in overweight women
TUESDAY, April 30, 2002 (HealthDayNews) -- Women who get pregnant while taking birth control pills might want to consider changing their diet before changing their method of contraception.
Reporting in tomorrow's Journal of the American College of Obstetrics and Gynecology, researchers spell out a possible relationship between Pill failure rates and weight, suggesting that women who weigh more than 155 pounds are more likely to experience Pill failure.
"Our finding suggests that perhaps women who weigh more may metabolize contraceptives differently, which in turn reduces their effectiveness," says study author Victoria Holt, an associate professor of epidemiology at the University of Washington School of Public Health in Seattle.
However, critics of the new research say there's little for overweight women to fear because even with the potential failure rate, the Pill remains among the most effect contraceptive available today.
"This study should not scare heavy women away from Pill use. The findings are preliminary, and the failure rates and the number of women affected was too small to consider stopping Pill use in this group," says Dr. Margaret Polaneczky, an obstetrician/gynecologist at the Iris Cantor Women's Health Center at the Weill Cornell Medical Center in New York City.
According to Holt, the study found that the more overweight a woman was while taking the Pill, the more likely she was to experience contraception failure. Additionally, heavy women who took low-dose birth control pills were especially vulnerable -- a finding that led researchers to believe failure rates could be linked to dosing.
Much the way doctors use a child's weight when deciding on how much medication to prescribe, Holt suggests using a woman's weight as a guide to what strength birth control pill would work best.
"What we are saying is that perhaps weight should be a consideration when deciding which Pill formulation to prescribe, with heavier women perhaps needing a higher dose pill than thinner women," Holt says.
High-dose birth control pills fell out of favor when studies showed they were linked to increased risk of blood clots, strokes and heart attacks -- health problems that can be exacerbated by excess weight.
Although Polaneczky says the dosing theory for heavier women might make sense, she's not convinced it's needed, at least not based on what the study shows.
"This is very preliminary data that raises an important question. But that question would require a lot further study before we can draw any conclusions about Pill use," Polaneczky says.
Holt's study gleaned data from 755 women who originally were controls for a larger study on ovarian cysts conducted between 1990 and 1994. At the time of the first study, the women answered detailed questionnaires and participated in a personal interview that touched on their lifetime history of a variety of demographic, medical and lifestyle factors, including contraception.
The original group of researchers, which was also lead by Holt, asked the women if they had ever been pregnant and counted only those conceptions that were verified by medical testing. The women were also asked to identify what, if any, type of contraception they were using at the time they conceived, and their height and weight at the time of conception.
The final study group consisted of 618 women who used birth control pills, with 106 pregnancies among them.
Using weight as a guide, Holt's new team of researchers divided the women into four groups, from the lightest (125 pounds or less) to the heaviest (155 pounds or more). Using the information the women provided on pregnancy and Pill use, the researchers calculated the overall pregnancy odds.
The result: Women in the highest weight group were slightly more than 1.5 times more likely to get pregnant while taking the Pill than the thinnest women in the group. Furthermore, heavy women who used low-dose birth control pills were up to 2.5 times more likely to get pregnant, while those taking the very low-dose birth control pills were 4.5 times more likely to conceive.
While most studies that test the Pill's effectiveness claim that failure rate is tightly linked to compliance, Holt says her study controlled for regular Pill use and found this did not differ among women according to weight. Additional analysis also revealed that women who had previously given birth, as well as first-time mothers, appeared to have the same rate of Pill failure.
Still, Polaneczky says the study should not scare heavy women away from Pill use.
"Even if larger studies validate this finding, we must remember that, even with diminished success rates, the Pill still remains more effective than other methods, regardless of a woman's weight. And it should not be discarded as a highly effective means of birth control," Polaneczky says.