NEW YORK (Reuters) The heavier a woman is, the more trouble she may have getting pregnant and having a baby through in vitro fertilization, or IVF and may lose the baby more often, according to a US study.Researchers led by Barbara Luke of Michigan State University found that women who were overweight or obese were less likely to become pregnant using fertility treatments than normal-weight women.Past studies have also hinted at worse IVF outcomes in heavier women, although they don't prove that the extra weight is directly responsible for the reproductive troubles those women experience."Treatment and pregnancy failures with increasing obesity significantly increased starting with overweight women," Luke and her colleagues wrote in Fertility and Sterility.They drew data from a reporting system that includes more than 90 percent of IVF treatments done in the United States information on 150,000 fertility treatment cycles done in 2007 and 2008 at 361 different clinics.For each cycle, the reporting system included whether the cycle was cancelled, if it led to a pregnancy, and whether that pregnancy ended early in a miscarriage or stillbirth, or if the woman gave birth to a live baby. For most cycles, it also had data on women's height and weight before starting treatment.From the beginning through the end of fertility treatment, heavy women saw poorer results."We know that being overweight and obese is not good (for IVF), it's just how bad is it and where are the bad effects?" said Brian Cooper of Mid-Iowa Fertility in Clive, who wasn't involved in the study.About nine percent of cycles in normal-weight women were stopped early, compared to 16 percent of cycles in the heaviest women those with a body mass index over 50, which is equivalent to a 5ft 5ins woman who weighs over 300 pounds.Normal weight women had a 43 percent chance of getting pregnant during each cycle using their own, fresh eggs for IVF, compared to 36 percent for very heavy women. Rates for overweight and less obese women fell in between.For women who did get pregnant, the trend continued, with the heaviest about twice as likely as normal-weight women to lose the baby in many cases.For overweight and obese women trying to get pregnant, even a little bit of weight loss helps, said Howard McClamrock, an infertility specialist at the University of Maryland Medical Center in Baltimore."This is what we're constantly faced with: ideally she might like to lose weight, but she might not have that much time," added McClamrock, who was not involved in the study.Though he noted that research has been pointing more and more towards a connection between extra weight and worse IVF outcomes, the reason is unclear.One explanation is that extra fat tissue releases estrogen, which fools the brain into thinking the ovaries are working when they really aren't, so it doesn't do its part to kick the ovaries into gear, Cooper said.Luke and her colleagues said that thin and heavy women may have different causes of infertility, though they added that they did not have data on lifestyle factors that may affect IVF success, or any data on the male partners.Thin and normal-weight women generally had higher rates of endometriosis, in which cells from the lining of the womb grow on other organs. Polycystic ovary syndrome, where the ovaries become enlarged and contain several small cysts, were more common in very heavy women.Cooper said that weight still isn't as big an issue for fertility as age, or whether a woman smokes."Weight isn't everything, but it's an important factor that we have control over. Fix it now, because even a little bit (of weight loss) can make a big difference," he added.