The odds of having a baby via in-vitro fertilization (IVF) may be lower for obese women than their thinner counterparts, two new studies find.

The studies, reported in the journal Fertility and Sterility, add to evidence suggesting that heavy women have a lesser chance of success with IVF -- where a woman's eggs are fertilized in a lab dish then implanted in her uterus.

Research shows that obese women may be less fertile than their thinner peers. But the evidence has been mixed on whether extra pounds can affect a woman's odds of having a baby with IVF.

In the new studies, researchers at two different Massachusetts fertility centers found that overweight women were less likely to have a baby after IVF.

In one, the birth rate among both overweight and obese women was 23 percent, versus 42 percent among women at the lower end of the normal-weight range.

In the other study, the odds of success were lower only for obese women, and not those who were less overweight.

Of 477 women who were moderately obese, 22 percent had a baby. That compared with 30 percent of normal-weight women.

And the chances of success dipped with the severity of a woman's obesity. Among the most obese women -- about 100 pounds or more overweight -- 15 percent had a baby.

The lead researcher on that study said there are still questions about the role of a woman's weight in IVF success.

In some past studies, researchers have found that normal-weight and obese women have similar chances of having a baby, said Dr. Vasiliki A. Moragianni, of Beth Israel Deaconess Medical Center and Harvard Medical School in Boston.

But most of those studies were much smaller than this one, he said in an email.

No difference for short-term weight loss

Moragianni's team had data on 4,600 women who underwent a first-time round of IVF or IVF plus intracytoplasmic sperm injection -- a technique used when there are problems with the sperm.

The other study, led by Dr. Jorge E. Chavarro, included 170 women who had at least one round of fertility treatment at the Massachusetts General Hospital Fertility Center.

Overall, obese and overweight women were less likely to have a baby, Chavarro's team found. So then the researchers looked at whether short-term weight loss made a difference.

There were 45 women who had managed to shed some pounds before starting their first treatment cycle -- typically about 6 pounds. But the researchers found no difference in their chances of having baby, even though those women did tend to produce a higher number of "mature," usable eggs.

None of that means weight loss is no use to heavy women undergoing infertility treatment, the researchers say.

The fact is, no one is sure what impact weight loss might have, according to Moragianni.

He said there is evidence that weight loss helps obese women who have a condition called polycystic ovary syndrome, which disrupts ovulation. But little is known about other women.

"It can certainly be hypothesized that weight reduction and lifestyle changes can improve (infertility treatment) outcomes in obese individuals," Moragianni said.

But clinical trials are needed to actually prove that is true, he added.

Researchers are not sure why heavier women might have less success with infertility treatment. Obesity may affect the quality of the eggs or embryos a woman produces, Moragianni said, and heavy women sometimes have a less "receptive" uterus versus thinner women. There may also be a role for hormones, since body fat can affect the ovaries' hormone production.