The heavier a woman is, the greater her odds against success with in vitro fertilization (IVF), suggests a large US study.
Women who were overweight or obese were less likely to become pregnant than normal-weight women – and when they did get pregnant, they lost the baby more often.
The patterns were especially clear for women using their own fresh eggs.
The new findings bolster earlier studies, which have hinted at worse IVF outcomes in heavier women. They don't prove the extra pounds are directly responsible for the reproductive troubles these women experience, but experts say that it's likely.
A look at the bad effects
"We know that being overweight and obese is not good for IVF, it's just how bad it is, and where the bad effects are," said Dr Brian Cooper, of Mid-Iowa Fertility in Clive, who wasn't involved in the new study.
To better get at that question, Dr Barbara Luke of Michigan State University in East Lansing and colleagues drew data from a registry that includes more than 90% of IVF treatments done in the United States.
In total, they had information on 150,000 fertility treatment cycles done at 361 clinics.
For each cycle, the reporting system included whether the cycle was cancelled, if it led to a pregnancy, and whether that pregnancy ended 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.
Heavy women likely to lose baby
From the beginning through the end of fertility treatment, heavy women had poorer results.
About 9% of cycles in normal-weight women were stopped early, compared to 16% of cycles in the heaviest women – those with a body mass index over 50.
Normal-weight women had a 43% chance of getting pregnant during each cycle, using their own, fresh eggs for IVF, compared to 36% for very heavy women. Rates for overweight and less obese women fell in between.
And 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.
Time could be a factor
“For overweight and obese women trying to get pregnant, even a little bit of weight loss helps," said Dr Howard McClamrock, an infertility specialist at the University of Maryland Medical Centre in Baltimore.
“However, this is what we're constantly faced with, ideally she might like to lose weight, but she might not have that much time," added Dr McClamrock, who was not involved in the new study.
He said research has been pointing more and more toward a link between extra weight and worse IVF outcomes. But, the real reason behind it is unclear, Dr McClamrock said.
One explanation, said Dr Cooper, is that extra fat tissue releases estrogens, 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.
Weight not biggest issue for fertility
Dr Luke and colleagues noted online in Fertility and Sterility that thin and heavy women may have different causes of infertility.
For example, thin and normal-weight women in the study generally had higher rates of endometriosis. Polycystic ovary syndrome was more common in very heavy women, however.
The researchers did not have data on lifestyle factors that may affect IVF success, or on any characteristics of male partners, including how heavy they were.
Dr Cooper, who was also not involved in the new study, said that weight still isn't as big an issue for fertility as age and whether or not a woman smokes.
“The findings should tell women that weight isn't everything, but it's an important factor that we have control over," he said. Fix it now, because even a little bit of weight loss can make a big difference.
(Reuters Health, August 2011)