A group of civil rights, health and women's organizations yesterday petitioned the Reagan administration to improve prenatal care for poor women, citing a new private survey showing that the mortality rate for black infants is almost twice as high as the rate for white infants.

The survey, taken in 45 cities between 1979 and 1981, indicated that the gap between black and white infant-mortality rates is widening.

The administrative petition charged that the administration is "determined to reduce federal spending or eliminate altogether" existing health-care programs that could reduce the infant-death rate.

"The administration's policy is not simply foolish," the petition to Health and Human Services Secretary Margaret M. Heckler said, "it is disastrous." The petition said savings from the program cuts in prenatal care "will soon be consumed by the tremendous costs" the government will pay in other health costs.

About two-thirds of the U.S. infant-mortality rate results from premature births and low-birthweight children who die in their first year.

Attorneys for the groups said the law requires the department to make a formal reply to the administrative petition's charges that HHS has been delinquent in implementing health-care programs already on the books. The lawyers held out the possibility that the groups might go to court if they weren't satisified with the answers they receive.