Pregnant women working at video display terminals more that 20 hours a week have an increased statistical risk of miscarriage during early pregnancy, according to a study, but the reason is a mystery.

The study by the Kaiser Permanente Medical Care Program's Division of Research, released this week, noted a "significant statistical relationship" between high VDT use and miscarriage in the first three months of pregnancy.

But the study "does not say what causes the problem," according to a Kaiser statement.

"VDTs themselves may be a cause, but stress, ergonomic factors - such as seating or eye strain - or unmeasured socio-economic factors might also be responsible for the increased risk," the statememt said.

"That VDTs themselves are hazardous to the pregnant operator remains a possibility," the study said.

The study involved nearly 1,600 pregnant women cared for at three Kaiser Permanente facilities between September 1981 and June 1982, Kaiser officials said.

The odds that heavy users of VDTs would miscarry were 1.8 times greater than low or non-users of VDTs, the study indicated.