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Weight May Affect Whether Birth Control Pills Work

OHSU researcher studies whether birth control pills are less effective in heavier women

Do birth control pills work differently depending on how much a woman weighs? That's what Oregon Health & Science University researcher Alison Edelman, M.D., wants to know. She is the principal investigator of one of the only studies in the country to evaluate the ovulation occurrence and medication levels of two groups of women using oral contraceptives: thin and plus-size women.

 "Almost all birth control efficacy studies have been done on average weight women. Most birth control pills contain a high enough dose to make sure women have enough medication to prevent pregnancy, but we don't have a lot of information about obese women. As the number of obese women increases this is important information to have. We want to know whether there is an actual biological reason for birth control failure in heavy women," said Edelman, assistant professor of obstetrics and gynecology, OHSU School of Medicine and member of the OHSU Center for Women's Health.

Oral contraceptives do not come in differing strengths according to a patient's weight, such as other medications. Several recent studies have shown a higher rate of birth control failure in heavy women. However, these studies were retrospective, not prospective studies in actual patients. Retrospective means a study starts with the present condition of a population and collects data about their past history to explain their present condition. A prospective study starts with the present condition of a group of people and follows them into the future.

Edelman is hoping to enroll about 20 thin and heavier women by summer 2006.

Heidi Milliken, a division manager for the OHSU Department of Obstetrics and Gynecology, and a thin woman, agreed to be in the trial. "I joined this study primarily for the purpose of supporting research in our community, but also to ensure that women are provided a contraceptive option that is going to be most effective. If there are indeed variances in the efficacy of a method due to one's weight, this is critical information that must be imparted to health care providers," she said.

 "My experience has been excellent. The study is well-designed; the principal investigator is very available to all questions; the exams and various tests have been fast, straightforward, and exactly as explained; and I'm getting a reliable method of contraception at no cost. The study coordinator has also done an excellent job of keeping me on track."

Participants are given breast and pelvic exams, a Pap smear (if a recent one has not been performed), a urine pregnancy test and a blood test to check for ovulation. The study will last for approximately 12 weeks.

Participants are required to have two 48-hour stays in a secure research unit to measure hormone and medication levels, and then several short outpatient visits. They are given free, low-dose birth control pills and must keep a daily diary of when they take their pills.

To be considered for this study, women must have a body mass index (BMI) of less than 25 for thin participants and more than 30 for plus-size. For example, a woman who is 5 feet 5 inches tall and weighs 130 pounds would have a BMI of 22; if that same woman weighed 180 pounds, she would have a BMI of 30.

To calculate your BMI visit: To participate in this study, please call the confidential recruitment line, 503 494-3666.

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