African-American women who undergo in vitro fertilization (IVF) are less likely to ultimately have a baby than are their white counterparts, a new U.S. study suggests.

Using data for more than 70,000 IVF treatment cycles performed nationally, researchers found that black women who underwent IVF using their own unfrozen eggs were less likely to become pregnant and more likely to miscarry than white women having the same procedure.

The birth rate per treatment cycle was about 19 percent among black women, versus 26 percent among white women.

There was, however, no racial disparity among women who underwent IVF using frozen embryos, with birth rates of 16 percent for white women and 16.5 percent for black women.

The researchers, led by Dr. David B. Seifer of Maimonides Medical Center in Brooklyn, New York, report the findings in the journal Fertility and Sterility.

Some previous studies examined the racial differences in IVF success rates, but have come to conflicting conclusions. To try to resolve the question, Seifer's team used data from a federal registry that collects information from fertility clinics nationwide; the researchers focused on 72,273 IVF treatment cycles performed between 1999 and 2000.

The majority were performed in white women, with African-American women accounting for about 3,700 of the treatment cycles.

The reasons for the lower success rate among black women are not clear, but the data suggest some possibilities, according to Seifer and his colleagues.

Black women, they note, tended to have more abnormalities of the pelvic structures, which can lower the likelihood of pregnancy and increase the risk of miscarriage.

Black women also tended to be older when they first sought IVF treatment and had fertility problems for a longer time — two more factors that dim the chances of success.

More studies are needed to understand why racial disparities in IVF success exist, according to the researchers. This is especially important, they note, in light of recent research showing that while infertility is becoming less common among white U.S. women, it is on the rise among African Americans.