CHICAGO, NOV. 19 -- Up to one in fifty pregnant women in America's inner cities may be infected with the AIDS virus, a rate as high as that in parts of Africa, where the disease is much more widespread, according to a new study.

Researchers led by Dr. Sheldon Landesman of the State University of New York Health Science Center in Brooklyn found that 2 percent of 602 women delivering babies at Kings County Hospital Center in New York had antibodies to the AIDS virus in the blood of their umbilical cords.

The presence of antibodies means the virus has entered a person's system and has provoked a natural disease-fighting response.

Almost half the women who tested positive said they did not know how they might have gotten the infection, suggesting that if blood samples had not been taken, their cases would have gone undetected, the researchers said.

No standard hospital testing programs exist for AIDS infections in pregnant women, even though the rate found in the study is "several times higher than that of many other diseases for which screening is already routine," the researchers said in Friday's Journal of the American Medical Association.

AIDS, or acquired immune deficiency syndrome, is believed to be spread mainly through sexual intercourse and the sharing of contaminated hypodermic needles. It can be passed from mother to fetus.

No one has reported percentages of pregnant women with AIDS antibodies at other U.S. hospitals, the researchers said. But the 2 percent rate found in the study is similar to one found in Nairobi, Kenya, though markedly lower than an 8 percent rate found in Kinshasa, Zaire, they said.

It is "reasonable to assume" that the prevalence in the New York City hospital is not unique to medical centers in that city or other areas where AIDS is commonly diagnosed in women, they said.