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Saturday 1st January, 2005
Getting pregnant while on the birth control pills: Yet another health hazard
for those that are overweight and obese.
SEATTLE ? Overweight and obese women who take oral contraceptives
are 60 percent to 70 percent more likely to get pregnant while on
the birth-control pill, respectively, than women of lower weight,
according to new findings from Fred Hutchinson Cancer Research
Center that will be published in the January issue of Obstetrics &
The study, led by epidemiologist Victoria Holt, Ph.D., M.P.H., a
member of Fred Hutchinson's Public Health Sciences Division, is the
largest case-control study of its kind to examine the link between
body-mass index and oral-contraceptive failure. The research was
conducted in collaboration with Delia Scholes, Ph.D., a senior
investigator at the Center for Health Studies at Group Health
Cooperative in Seattle.
"The results of our study represent yet another reason why
obesity is a health hazard," Holt said. "Overweight and obese women
have a significantly higher risk of getting pregnant while on the
pill than women of normal weight, and this translates into a
substantial number of unplanned pregnancies." Among 100 women taking
oral contraceptives for a year, Holt's study suggests that an
additional two to four women will get pregnant due to being
overweight or obese.
"This higher risk of pregnancy also translates into a higher
number of obesity-related complications of pregnancy, which range
from gestational diabetes and high blood pressure to Cesarean
delivery," Holt said.
Body-mass index, or BMI, is a measure of body fat based on height
and weight. The World Health Organization divides BMI into four
categories for both men and women: underweight (18.5 or lower),
normal (18.5 to 24.9), overweight (25 to 29.9) and obese (30 or
greater). A BMI calculator is available
"We found little difference or variation in the risk of
contraceptive failure among women who fell into the first three
quartiles of body-mass index, but the risk jumped up dramatically
among women in the fourth quartile," said Holt, also a professor of
epidemiology at the University of Washington School of Public Health
and Community Medicine.
Holt and colleagues found the association between extra pounds
and pill failure first surfaced among overweight women whose
body-mass index was 27.3 or higher, which is equivalent to a 5-foot,
4-inch woman who weighs 160 pounds or more. These women faced a 60
percent greater risk of getting pregnant while on the pill. Those
considered obese, with a BMI of 32.2 or greater, faced a 70 percent
While no one knows for sure why overweight and obese women
experience a higher degree of oral-contraceptive failure than their
thinner counterparts, several biological mechanisms could account
for the effect.
One possible explanation is increased metabolism. "The more a
person weighs, the higher their basal metabolic rate, which can
shorten the duration of a medication's effectiveness," she said.
Another possibility is that the heavier a person is, the more liver
enzymes they have to clear medications from the body, causing a drop
in circulating blood levels of the drug. A third theory is based on
the fact that the active ingredients in oral contraceptives ? the
hormones estrogen and progesterone ? are stored in body fat. "The
more fat a person has, the more likely the drug is sequestered, or
trapped, in the fat instead of circulating in the bloodstream," Holt
Another factor that may account for obesity-related pill failure
is that today's oral contraceptives contain relatively low hormone
levels compared to those first introduced four decades ago. "Hormone
levels have decreased fivefold in an effort to reduce unwanted side
effects ranging from weight gain to risk of blood clots and stroke,"
Holt said. "Today's pill dose is high enough for most women but may
not be adequate for all."
So does this mean that overweight women who don't want to risk an
unplanned pregnancy should demand a higher-dose pill? Not
necessarily, Holt said.
"Overweight women are more likely to have cardiovascular-disease
risk factors than normal-weight women, and to increase the dose of
contraceptive hormones might increase cardiovascular risks even
more," she said.
Instead, Holt recommends that overweight women who have completed
childbearing consider a permanent form of birth control such as
sterilization, and that overweight women who have not yet completed
childbearing consider using a backup form of birth control while on
the pill, such the condom or other barrier methods.
Another recommendation for heavy women on the pill is to lose
weight, although this study did not specifically examine the impact
of weight loss on the pill's effectiveness. "I think losing weight,
if one is substantially overweight, is a terrific idea for many
health reasons and a laudable goal," Holt said. "I also acknowledge
that it is often difficult to do."
For the study, Holt and colleagues compared weight and body-mass
index of 248 women who became pregnant while on the pill to an
age-matched comparison group of 533 nonpregnant oral-contraceptive
users. All of the study participants were enrolled in Group Health
Cooperative, a Seattle health-maintenance organization. The women's
self-reported history of weight, pregnancy and oral-contraceptive
use was verified through medical, laboratory and pharmacy records,
which added statistical strength to the study's findings, Holt said.
Fred Hutchinson Cancer Research Center.
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