When a Fairfax County housewife, already the mother of four recently decided to have an abortion, she contacted a Washington area abortion service and was told the cost would be $140.

The woman, like millions of other Americans, had medical insurance. After the abortion, the doctor sent her insurance carrier, Prudential, a bill for $410.

"I was just infuriated when I realized what had happened," said the woman, Stephanie Covington, 32, who received a copy from the insurance company of the check it had sent to Dr. De Dinh Tran, the physician who performed the operation at the Abortion and Complete Family Planning Services office located in the Skyline Plaza in Falls Church.

Covington's experience is repeated in clinics and doctor's offices around the country, according to health and insurance experts. They warn that the practice of charging a higher fee for patients with insurance coverage is a major reason for soaring insurance premiums and medical costs.

"It is a classic problem," said Sidney Wolfe, director of the consumer-oriented Health Research Group. "Some surgeons actually brag about how they charge more to insurance-covered patients. And there's nothing illegal about it."

Although Wolfe cited another case of an abortion clinic in Baltimore that charges Blue Shield $350 and noninsured patients $165, he said that abortion are by no means the only medical services that operate under two-price systems.

Because insurance companies pay claims based on the prevailing rate that doctors charge in a particular geographical region, "their's a built-in incentive to raise fees, and the cost keeps going up and up," Wolfe complained.

In the Washington area, for example, insurance firms regard $450 as the average cost of an abortion. That figures is based on a sampling of abortion fees charged by physicians and makes no distinction between abortions performed in clinics and those performed in a doctor's private office.

Thus, according to a Prudential spokesman, the firm did not balk when it received a bill for $410 from Dr. Tran.

"If we had known he was quoting a price of $140, that's all he would have gotten," said Clifford Hoch Jr., a Prudential vice president. "But if an individual health care provider has no ethics, he can charge whatever he wants up to the usual and customary limit before we'll raise any questions."

Covington and Dr. Wolfe argued that the Prudential reimbursment was all the more galling because most abortions performed in the first trimester of a pregnancy are relatively inexpensive.

"The highest price I was ever quoted over the phone was $175," said Covington, echoing Wolfe's statement that a $350 to $450 insurance reimbursement is "'way out of line" with what regular clinics charge.

At the Falls Church facility, Dr. Tran and his office assistant, Ngoc Tsi, said abortion costs there depend on a patient's ability to pay and on the method of payment.

"If you bring cash, it's $140," Tsi told another person who called the office. "If we bill, it's more." She said the regular cost of an abortion is $350 "unless the person is poor and cannot afford to pay."

She and Dr. Tran denied they had billed Covington or other patients more than $140 just because they had insurance. The $140, they said, "was just a deposit."

Covington said the woman who answered the phone at the office never asked for any fee higher than $140 and never asked about her ability to pay.

"I called twice, and both times they told me to bring a check for $140," she recalled. "They never mentioned any other amount or suggested it was a deposit -- if they had, I would have gone to where I could get the abortion for $175."

Although Covington found Dr. Tran's Falls Church office listed in the telephone book under the name, Abortion and Complete Family Planning Services, the Vietnamese physician said yesterday that he rarely performs such operations. He said his specialties are acupuncture, obstetrics and family planning, which he has practiced since coming to the United States 10 years ago.

Since abortion clinics must be licensed by the state, a Virginia health official said he is checking into Dr. Tran's practice to see if he is operating an abortion clinic rather than an office service as Tran claims. If the state decides the doctor is operating a clinic out of his office, which Tran denies, Tran could encounter additional difficulties: he is not a board certified gynecologist, and state law requires that abortion clinics have a certified specialist on their staffs.

Covington said her suspicions were first aroused when she showed up at Tran's office and was told they would keep her check while they processed the insurance forms. The woman filling out the forms began to include several additional charges for laboratory tests until Covington told her none had been performed, Covington said.

The only unusual aspect of her case, Covington said, was her need for a special shot to counteract an Rh negative blood condition. Even then, she was told the shot would cost $25 but later discovered Prudential had been billed $60. This brought the total bill to $410.

Prudential paid $335.50, a fixed percentage of the final bill, according to the terms of Covington's policy.

Covington, whose husband notified the insurance company of the discrepancy, said she felt those who operate the clinic are taking advantage of the situation.

"Besides," she added, "if the billing system is really legitimate, why didn't I get a bill for the difference between what they charged and what Prudential paid?" Her original check for $140 was returned to her, and she said she never has been asked to pay the remaining amount of the $410 billed to the company.

"They probably figured I'd never say anything about it," said Covington, who was six weeks pregnant when she had the abortion.

According to Prudentials' Hoch, "more often than not, if an individual doesn't have to pay anything, they never tell you anything. We send out copies of the bill to make them aware of it, but it's rare that it's brought to our attention."

The insurance company says it is investigating the case and will be contacting Dr. Tran.

Max Fine, executive director of the Committee for National Health Insurance, said inflationary billing and reimbursements "happen all the time. Health insurance is inflationary, and when a third party pays, people don't argue about the costs."

Arguing that insurance benefits should be prenegotiated so that doctors and hospitals would be encouraged to hold costs down, Fine said the present insurance system -- subsidized by business and industry -- keeps driving costs up.

"When you have insurance, the cost is double," he said. "Chrysler in this country pays $2,500 per worker for health benefits, but Chrysler in Canada pays one-third that."