The nation could prevent nearly a third of all genetically caused retardation by the year 2000 if it would move faster to mass-screen newborns and examine women in pregnancy, three leading geneticists said yesterday.

They told the American Association for the Advancement of Science that 160,000 to 170,000 pregnant women a year - mainly those 35 and older at higher risk of bearing a retarded child - should be offered the possibility of amniocentesis, or diagnosis of possible defects by examing the fluid in their wombs.

Only 20,000 women a year are getting the test, "an astonishly slow" development, said Dr. Aubrey Milunsky of Harvard Medical School and the Eunice Kennedy Shriver Center in Waltham, Mass.

Only 14 states do at least four of a possible eight or nine blood tests on newborn babies for treatable genetic and chemical disorders, said Dr. Robert Guthrie of the State University of New York at Buffalo.

Milunsky, Guthrie and Dr. Maurice Mahoney of Yale University blamed the lag in using these techniques largely on "ignorance" - a word they all used - by parents, legislators, the general population and doctors.

They said there is also of financial support, though France has found that for every franc spent on blood testing alone, eight francs spent on the future care of defective children are saved.

Also, Milunsky said, there is growing opposition to amniocentesis by anti-abortionists. This is because parents who are told a fetus is defective often choose to end the pregnancy by abortion.

"Amniocentesis is really a life-saving, not a life-destroying technology," Milunsky said. "Far more babies have been born because of amniocentesis - to women who otherwise would never have dard become pregnant for fear of some defect that ran in their families - than those not born because of abortion."

There is no national policy among geneticists to favor abortions, he said, only a policy to the effect that "everyone has a right to the information" about their unborn children if they want it. "What happens then is up to the individual," he said.

The risk of one common defect alone - the genetic disorder that causes mongolism or Down's syndrome - is one in 100 in women 35 and older, compared to one in 2,000 to 3,000 up to age 25, and one in 1,000 in women 30 to 34.

Guthrie - developer of a test for one chemical disorder, phenylketonuria or PKU> now used in every state - said the following states do "at least half" the tests that can be done today on the same blood sample taken for PKU testing: New York, Maryland, Ohio, South Carolina, a New England regional group (Massachusetts, Maine, Rhode Island, Connecticut, New Hampshire) and a northwest group (Oregon, Alaska, Idaho, Montana, Nevada).

Among the regional groups - with all the tests done in one state laboratory (in massachusetts for New England and Oregon for the northwest), the test cost has been brought down to "mere cents," Guthrie said. In most other states, he said, small numbers of tests are done in several private or hospital laboratories, "with a few dollars profit added for every test."

Milunsky said "one of the most important steps the country culd take" would be extension by Congress of the National Genetic Disease Act passed in 1976, but only now being implemented with an initial $4 million in planning and organization money. The act expires this fiscal year unless Congress renews it.