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

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

British doctors tracked all 318 women at a London clinic who became pregnant after having in-vitro fertilization from 2006 to 2009, then divided the women according to their Body Mass Index. Women who had a BMI of 18 to 24 were classified as normal. Those with 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.

The research was presented yesterday in Rome at a meeting of the European Society of Human Reproduction and Embryology.

Among women with a normal weight, 22 percent using in-vitro at the clinic had a miscarriage. But among overweight and obese women, the risk of a miscarriage was 33 percent.

For women who have conceived naturally, the miscarriage rate can range from 4 percent to 23 percent during the first trimester, depending on their age and medical history. Experts say the risk for overweight and obese women conceiving naturally can be three to four times higher than the average for regular-weight women.

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

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 cutoff is often lower, at a BMI of 30.

Doctors aren't sure why excess body weight makes pregnancy more risky, but suspect that fat may have harmful effects on the lining of the uterus, making it difficult for embryos to implant properly.

"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.

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