Asian-American women may be less likely than white women to successfully have a baby after undergoing in-vitro fertilization, a new study suggests.

Researchers at Stanford University in California found that of 180 women who underwent IVF at their fertility clinic, women of Asian descent were less likely to have a successful pregnancy: 31 percent gave birth, compared with 48 percent of white women.

The findings, reported in the journal Obstetrics & Gynecology, add to evidence of racial disparities in IVF outcomes. Past studies have found that Asian- and African-American women have lower success rates than their white counterparts.

The reasons for those racial gaps remain unclear, however.

This latest study does not answer the question of why Asian Americans are less likely to give birth after IVF. But it does offer evidence against some of the proposed explanations, say the researchers, led by Dr. Elizabeth S. Langen.

The researchers found no evidence, for example, of racial differences in the quality of the embryos produced by IVF. Asian-American women were also similar to white women in their response to the medications used to stimulate the ovaries to produce eggs; in both groups of women, doctors were able to retrieve an average of 15 to 16 eggs.

The findings suggest that “factors other than embryo development influence IVF outcome in Asian women,” Langen and her colleagues write.

The study included 180 women who underwent an IVF cycle using their own eggs in 2005 or 2006. Sixty-two percent were white, and 38 percent Asian.

Overall, Langen’s team found, there were no clear racial differences in the initial part of treatment. The women had similar responses to ovarian stimulation, their rates of successful fertilization were comparable, and the number of embryos implanted in the uterus were similar - two, on average, in both groups of women.

But Asian women were less likely to become pregnant - 43 percent did, versus 59 percent of white women - or to ultimately have a baby.

Weight is a factor in IVF success, with both obesity and excessively low weight linked to lower success rates. In general, Asian-Americans were thinner than white women, Langen and her colleagues note, but women in both groups were typically within normal weight range. Only three Asian women were underweight.

Another theory on why Asian women have lower IVF success rates is that they may, as some research suggests, have a higher rate of endometriosis - a disorder where the tissue lining the uterus also grows outside the uterus, often affecting the functioning of the uterus and ovaries.

Langen and her colleagues did not assess the women for endometriosis, but they also found no racial differences in the rates of past endometriosis diagnoses.

The researchers point out that they had no information on lifestyle factors like diet or supplement use. Future studies, they conclude, should look at whether racial differences in those factors play any role in IVF outcomes.

SOURCE: Obstetrics & Gynecology, March 2010.