Seventy children a year can be expected to develop cancer because their mothers were needlessly X-rayed during pregnancy, witnesses told a House health subcommittee yesterday.

The procedure they described is called "X-ray pelvimetry," and it was once widely used to assess mothers' ability to deliver their babies.

Ultra-sound has now largely replaced it. But it is still being used before six of every 100 deliveries in this country, Dr. Leon A. Phillips, associate professor of radiology at the University of Washington Medical School in Seattle, estimated.

Based on the average X-ray dose used - and on National Academy of Sciences calculations of the effects of radiation on the fetus - this means that either cancer or leukemia could be expected to develop each year in about 70 children whose mothers had pelvimetries.

The only remaining role for X-ray pelvimetry" is in preparing for a breech delivery, Phillips maintained. "This means the number could be cut nearly to zero and certainly far less than 1 percent," he said.

At his university hospital, "approximately 100 to 200 pelvimetries" used to be done each year, "mostly by insecure residents in training seeking assurance and believing it was necessary to avoid possible litigation," he reported.

But now, he said, every such procedure must be approved by a senior physician, and "this policy has all but eliminated" the practice.

Also, he said, the Washington State Professional Standards Review Organization - a federally mandated doctors' organization - is starting to educate all doctors to be as careful. Phillips directs that group's program.

He was one of a series of witnesses on over-use of X-rays in almost every part of the body. "The over-utilization of X-rays for diagnostic uuse alone is estimated to cost the public $2 billion a year, quite aside from the health implications," said subcommittee chairman Paul Rogers (D-Fla.).

Radiation exposure standards in industry also came under attack yesterday. Dr. Eula Bingham, assistant secretary of labor for occupational safety and health, called them "not adequate" and they "should be reduced."

The standard for workers currently is five rem per year, but that level is being challenged by some doctors and scientists who claim long-term exposures at even lower levels have increased the risk of cancer.

The National Academy of Sciences has a committee studying the effects of radiation with an eye toward recommending new exposure standards.

The chairman of that committee, Dr. Edward Radford of the University of Pittsburgh School of Public Health, recently told a House subcommittee that he thought the annual worker exposure level should be cut tenfold - to .5 rem a year.

Applying Radford's suggested standard. Public Citizen, the Washington-based health research lobby, reviewed Nuclear Regulatory Commission figures on 1976 worker radiation levels in atomic power plants. The study found that one in five exceeded the .5 rem level.

Citing that information, Public Citizen's director, Dr. Sidney Wolfe, in a letter to OSHA's Bingham urged support for "lowering the allowable worker exposure to .5 rem per year."

Bingham's statement yesterday was in response to Wolfe's letter. Although Bingham noted "many leading authorities recommend reducing the standard to .5 rem," she committed herself only to working "closely" with Energy Secretary James E. Schlesinger and his staff "to develop a program for correcting the presenting situation."