LONDON (AP) -- Overweight women have a much higher risk of a miscarriage after having in-vitro fertilization compared with slim women, new research says.

British doctors tracked all 318 women at a London clinic who became pregnant after having in-vitro fertilization from 2006 to 2009. They divided the women according to their Body Mass Index. Women who had a BMI of 18 to 24 were classified as normal. Those who had a BMI of 25 or above were considered overweight, while those above 30 were obese.

After making a statistical adjustment for factors that might have skewed the results -- such as age, smoking and medical history -- the researchers found overweight and obese women were much more likely to have a miscarriage as thin women. The research was presented Monday in Rome at a meeting of the European Society of Human Reproduction and Embryology.

Among women with a normal weight, 22 percent had a miscarriage. But among overweight and obese women, the risk of a miscarriage was 33 percent.

Doctors have long known that heavy women are more prone to have a miscarriage and suffer other complications after becoming pregnant naturally. But there has been conflicting data over whether that was also the case after using artificial reproduction techniques.

"One of the best fertility treatments is weight loss," said Dr.Richard Grazi, director of reproductive endocrinology and infertility at Maimonides Medical Center in New York, who was not linked to the study.

"With the obesity epidemic, we are constantly counseling patients to lose weight if they want to have IVF," he said. "Our goal is not only that they get pregnant, but that they stay pregnant."

At Grazi's U.S. clinic, patients with a BMI above 35 are not eligible for in-vitro fertilization. At many hospitals across Europe -- where at least one cycle is paid for by governments -- the cut-off is often lower, at 30.

Doctors aren't sure why excess body weight makes pregnancy more risky, but suspect fat may have harmful effects on the lining of the uterus, making it difficult for embryos to be properly implanted. "Fat doesn't just sit there, it's very dynamic and sends out hormones and other signals that affect the rest of your body," Grazi said.

"Our aim was not to exclude women from getting treatment, but to help women get the best outcome after they have IVF," said Dr. Vivian Rittenberg, a clinical fellow in the Assisted Conception Unit at Guy's and St. Thomas' Hospital in London, who led the study. She said overweight and obese women pursuing artificial reproduction techniques should get more help losing weight first.

Other experts agreed the findings should spur heavy women to drop a few pounds before pursuing in-vitro fertilization, a costly and invasive process that can last for months. Only about one in four couples pursuing IVF end up with a baby, and being fat can reduce those odds even further.

"Overweight and obese women have poorer outcomes at nearly every stage of pregnancy," said Dr. Daghni Rajasingam, an obstetrician and spokeswoman for Britain's Royal College of Obstetricians and Gynaecologists.

Rajasingam said overweight or obese women are also at higher risk for developing diabetes while pregnant, which in turn raises the chance of a miscarriage.

She said women should realize losing weight could make a big difference to having children.

"If you optimize your body weight so that your BMI is normal, your chances of having a baby to take home are much higher," Rajasingam said.

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