Susan Hill is explaining to a phone caller why she pays thousands of dollars a year to fly in out-of-state doctors to work in her Fargo, N.D., abortion clinic _ no doctor in North Dakota wants the job, she says _ when she is called to another phone. A minute later, she is back. ``Our Indiana clinic was just calling about a bomb threat,'' she says.

Abortion is one of the great dividing issues of our time, a moral battleground and a political hot potato. But it is also something more: a $450 million business. Each year, roughly 1.5 million abortions are performed at an average cost of $300 each. Ninety percent are performed by independent clinics, many of them for-profit, as are the nine clinics that constitute Ms. Hill's National Women's Health Organization chain.

In some ways, the Fargo facility is atypical of the nation's mostly urban abortion clinics. Perched alone at the edge of the prairie, its clients include farm women who often drive for hours to reach it. The Fargo clinic's violent history, including two firebombings, also sets it apart. Flip Benham, director of Operation Rescue, the antiabortion group, calls Fargo ``the one mill in North Dakota, period. And we are looking for one state to finally be abortion-free.''

Nor has isolated Fargo been hit with the same declines that have affected the abortion industry generally, says Ms. Hill, who is 46 years old. By 1992, the latest statistical year, abortions nationwide had fallen to the lowest levels since 1979 _ 1.5 million _ because of demographic and attitudinal changes, including society's increased acceptance of unwed mothers. The resultant cutthroat competition among abortion clinics for that business is so fierce that the National Coalition of Abortion Providers, a trade group, has added a popular ``Marketing Tips'' column to its newsletter.

But Fargo illustrates in broad terms some of the business issues facing abortion providers _ primarily, the substantial and fluctuating costs associated with pressure from abortion foes. Ms. Hill says her company, which is based in Raleigh, N.C., and has centers in eight states, is profitable overall. But last year, Fargo, a perpetual loser, lost another $16,000 on revenue of $500,000. Other centers around the country are also struggling financially, according to one industry trade group. Neither Ms. Hill nor other abortion providers will say how profitable their businesses are, in part because they fear retaliation.

Of the Fargo clinic's 1994 revenue, 25 percent came from routine gynecological procedures and the remainder from abortions, for which the clinic charged $400 for a first-trimester operation. But offsetting revenue were annual expenses that include the $100,000 paid to her chief doctor for working two days a week and $11,000 for two armed guards to roam the clinic on days when abortions are performed. Security expenses would have been higher, Ms. Hill says, if U.S. marshals hadn't stepped in to help five months ago.

Before 1992, Fargo's security costs were negligible because Ms. Hill relied on volunteer escorts. In 1993, alarmed by the abortion movement's first murder, her security budget mushroomed to $30,000, as she outfitted her three doctors with $750 bulletproof vests and installed door buzzers and surveillance cameras. This year, she is already anticipating a doubling of the 1994 security budget to cover a $3,000 metal detector, plus an operator's salary, in the wake of the two Brookline, Mass., abortion-clinic slayings in December.

Like security costs, legal bills fluctuate wildly. Three years ago, Ms. Hill paid $50,000 in the aftermath of demonstrations by the militant Lambs of Christ and other groups. Last year, she paid $23,000 _ ``that was just for ongoing things, like when people are chained to the building.'' At any given time, the clinic is embroiled in anywhere between one and four legal actions with protestors, says her lawyer, William Kirschner.

``Every issue, they make crazy motions,'' he says. ``They argue about everything. They are basically trying to bankrupt the clinic.'' Recently, for example, Ms. Hill says she was forced to take several men to court for trying to block Fargo's entrance by attaching themselves with bicycle locks to junk cars.

Fargo's property and casualty insurance costs hovered at $2,000 for 1994, about the same as the previous year, despite two incidents of arson and a vandalism spree by abortion foes that left the clinic covered with scrawled slogans. Although Ms. Hill says she doesn't turn in claims under $5,000, she says that Fargo's insurance has been canceled several times over the years; she won't provide the names of her current insurer.

While she has never had trouble finding an insurance replacement, other providers have, particularly with big companies. For example, Travelers Inc.'s Travelers Insurance Co. says it stopped underwriting abortion clinics in 1990.

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