Bryna Harwood

(WOMENSENEWS)–Most contraceptive failures are chalked up to inconsistent or incorrect use. However, not much is known about the characteristics of women for whom contraceptives fail. Recently, medical researchers have begun to probe whether a woman’s weight can be a factor in the reliability of her birth-control method.

More than 10 million women in the United States use oral contraceptives, making them themost common reversible method of birth control.Although these pills are popular, reliable and well tested, they are not infallible. About half a million women using the pill become pregnant each year.

Researchers have found that body weight may affect metabolism sufficiently to compromise contraceptive effectiveness. Victoria Holt, an epidemiologist at the University of Washington, categorized women who filled out a health survey into four groups by weight. Women in the highest body weight category–above 155 pounds–had a 60-percent increased risk of oral contraceptive failure compared with all women of lower weight, according to Holt’s 2002 study published in the journal Obstetrics and Gynecology.

Study Finds Possible Weight Factor

When Paul Norris examined the medical records of 514 women using low-dose oral contraceptives at a reproductive-health clinic in Miami in 2002, he found seven cases in which the patient became pregnant. Of those, five were overweight or obese.

"Overall the pill’s failure rate was low, which is consistent with use of birth-control pills," says Norris, a gynecologist at the University of Miami in the division of obstetrics and gynecology. Perfect pill use results in a failure rate of less than 1 percent, actual failure rates run about 6 percent. Most unintended pregnancies among women using contraceptives occur because of imperfect compliance with their pill-taking regimen, for example, missing a daily pill or taking the pills at different times of day.

To test for a relationship between weight and oral-contraceptive failure, Norris calculated women’s body mass index, or BMI. While not a perfect measure for fat, BMI is easy to calculate by using height and weight. Women with a BMI between 25 and 29 are considered overweight and those with a BMI 30 and above, obese. In Norris’ study, overweight women with a BMI greater than 25 were almost three-times more likely to experience birth-control-pill failure. Obese women had a failure risk that was more than five times that of all other women in the study, including those who are overweight.

Norris study was too small to draw any firm conclusions, but it charts enough of a trend to warrant further investigation. "I’m hoping to recruit other centers that have data available so we can look at 5,000 women," he says.

Hormones Are Fat Soluble

Experts are not sure why overweight or obese women experience larger failure rates on the pill.

"Hormones like those in the pill are fat-soluble," says Norris, "so they may disappear into the fat and not prevent contraception as well as they might on a thinner person. Put simply, more fat means more hormones are bound up or unavailable to work on the ovaries."

But, she says, this does not necessarily mean the pill is less effective for overweight and obese women.

"We can’t draw that conclusion at the moment." says Norris. "Even if you have a 5.6 percent risk of pregnancy, you still have 94.4 percent effectiveness and that’s pretty decent compared to some contraceptive methods."

Bryna Harwood, assistant professor of obstetrics and gynecology at the University of Pittsburgh, is another authority on contraceptives. She says researchers should also investigate lifestyle or other factors–beyond hormonal
–that may make an overweight woman’s contraception less reliable.

Many Variables in Birth-Control Reliability

The effectiveness of the pill is lower in certain groups of women, she says, but for reasons that have nothing to do with weight. For example, teen-agers may have higher oral-contraceptive failure rates because their schedules are more erratic or they may not be able to leave their pills out in the open, in a place where they can remember to take them. Both these factors could affect pill-taking consistency.

"It’s not the hormones in teen-ager’s systems that are responsible for the failure," says Harwood. "Maybe women with other issues–like being overweight
— have patterns or behaviors that make it less easy to take a pill everyday. I don’t mean to say that’s true, just that there may be explanations beyond strictly pharmacologic ones."

Weight is not a factor with the intrauterine device or barrier methods such as the diaphragm, or with the vaginal ring, according to the U.S. Food and Drug Administration, the federal agency charged with assuring safety and effectiveness of consumer products.

Patch’s Effectiveness Compromised by Weight

One method that is clearly impaired by weight is the contraceptive patch. Released in 2002, the once-a-week Ortho Evra patch has pregnancy-prevention rates equivalent to the pill–99 percent. However, in clinical trials, the patch wasn’t as successful for women over 198 pounds–only about 92 percent effective.

"For those women the effectiveness rate was worse than that reported for condoms," says Harwood from the University of Pittsburgh. "Further studies need to be done to see if placing the patch in a different spot on the body or a different dose might be better for overweight women." Typically, the patch is placed on the buttocks, abdomen, upper torso (excluding the breasts) or upper outer arm. Patches are applied weekly for three weeks and provide a stable dose of the hormones found in most birth control pills. Women forgo the patch during the fourth week and have a menstrual period.

The patch’s prescribing information contains a precaution that Ortho Evra may be less effective in woman weighing more than 198 pounds than in women with lower body weights. (Ortho Evra is the brand name for the contraceptive patch produced by Ortho-McNeil Pharmaceutical, Inc., headquartered in Raritan, N.J.)

"I do counsel overweight women differently based on efficacy studies of the contraceptive patch," says Harwood, "but prescribing practices shouldn’t be changed based on any of the literature that has looked at weight and oral contraceptive use so far. In general, I consider weight along with all other aspects of a woman’s health history, to reduce unintended pregnancy in the best way for her."

Kathleen Nelson writes about health, science, and medicine for general and specialty audiences.



For more information:

Obstetrics and Gynecology 2002;99:820-827—
"Body Weight and Risk of Oral Contraceptive Failure":

Association of Reproductive Health Professionals: