Obesity has long been associated with infertility as well as lower success rates with in vitro fertilization, and now researchers think they understand why: Obese women are more likely to have abnormalities in their eggs that make them impossible to fertilize.

Brigham and Women’s Hospital infertility researchers examined nearly 300 eggs that failed to fertilize during IVF in both severely obese women and those with a normal body weight. They found that severely obese women were far more likely to have abnormally arranged chromosomes within their eggs compared with women who weren’t overweight, according to the study published Wednesday in the journal Human Reproduction.

Researchers still don’t completely understand why obesity might lead to structural problems in eggs, but it could have something to do with higher levels of hormones like leptin, which make chromosomes more fragile.

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Previous research has linked similar abnormalities to aging eggs, which could at least partly explain why women over 40 have a much harder time getting pregnant than those in their 20s.

While women can’t do anything to turn back the clock, they may be able to improve their egg quality by achieving a healthier weight, said Dr. Catherine Racowsky, study co-author and head of the Brigham’s IVF lab. “Many obese infertility patients in our program wind up achieving successful IVF after they lose weight,” she added.

The women in the study were severely obese with a body mass index of at least 35—or 210 pounds for a 5-foot, 5-inch person—and it’s likely that moderately overweight women who carry just 20 or 30 extra pounds don’t have the same increase in egg abnormalities.

But those at the extreme end of the weight spectrum who are attempting infertility treatments should make an effort to nudge their weight closer to a healthier BMI. Obese women who lose just 5 or 10 percent or their body weight may see enough improvement, according to Racowsky, to help them achieve a successful pregnancy with IVF or natural methods. By the same token, underweight women, who have a BMI under 18.5—or 111 pounds for a 5-foot, 5-inch person—are also at risk of infertility and might increase their pregnancy odds by gaining a few pounds.

“We’d like to see if underweight women have the same increase in egg abnormalties in future research,” Racowsky said, “and also to see how much these abnormalities occur in those who are moderately overweight.”