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Researchers compared the medical records of 223 women aged 15 to 44 who were using two different types of IUDs, following them for up to two years later.
Women in both groups appeared to lose about 1 percent of their body weight in the first and second years of having an IUD.
The study was scheduled to be presented Monday at the American College of Obstetricians and Gynecologists annual meeting in San Diego.
"We really expected to see weight gain, and we didn't even expect that there would be weight loss," said study author Dr. Erika Kwock, an obstetrician and gynecologist at Kaiser Permanente Northern California in Santa Clara.
Although previous research has not found associations between hormonal or non-hormonal IUDs and weight gain, Kwock thought that the women in her study would put on pounds "just because over time people tend to gain weight regardless of contraception," she said.
However, Kwock pointed out that the weight loss among the women in her study is probably not a reliable result. Her study did not include enough women to allow for a statistical analysis to show that the women actually shed pounds.
Still, "the numbers are encouraging that there is not a weight difference for LNG IUDs and copper IUDs," Kwock said.
Dr. Jill Rabin, head of urogynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y., said, "This study is interesting because it opens the door for more questions and more research."
But there were not enough women in the study to know if the IUDs were associated with weight changes, Rabin added.
"People are always worried about weight gain whenever you mention a hormone, even though weight gain is miniscule in our experience," Rabin said. "But I don't think we've answered the question, certainly not with the LNG, and not even with oral contraceptives."
Many women in the study received Kaiser insurance through work, Kwock said. Otherwise they were diverse and probably representative of the women nationwide, she added.
Kwock and study co-author Dr. Julie Livingston looked at a number of factors in the Kaiser medical records of these women, such as weight, age, race, medical conditions that might cause them to gain weight (such as diabetes and thyroid disease) and whether they were taking an antidepressant.
They found no differences for any of these factors between LNG and copper IUD users.
In addition, weight loss, albeit small, seemed to be similar between the LNG and copper IUD users; however this result might not be real, Kwock again cautioned.
Many women start on an IUD after they have had a baby, so Kwock and Livingston compared the proportion of women in each group who had received their IUD within two months of childbirth but found no differences.
The weight loss that the researchers saw in each group was not just due to the fact that some of the women were losing their "baby fat", Kwock said.
"A lot of the doctors we work with really recommend IUDs for new moms because they are busy and they don't have time to remember to take pills," Kwock said.
"One of my favorite forms of birth control is the LNG because it has so many benefits -- women get lighter periods and have less cramping, while the copper IUD can actually make periods more heavy," Kwock said.
However, some patients prefer copper IUDs and it really depends on the patient, Rabin said.
More than with IUDs, women really worry about gaining weight on the pill, and while this cannot be ruled out, most research does not find this to be the case, said Laureen Lopez, a family planning researcher at FHI 360 in Durham, N.C.
"We've concluded that women need to have more appropriate counseling. A lot of people unfortunately gain weight over time, and you need to look at dietary patterns and exercise and not blame a contraceptive for which there may be little evidence," Lopez said.
Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
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