NEW YORK (CNN/Money) - Shares of American International Group, a stock that for decades outperformed the market, have stumbled this year, falling more than 26 percent in three weeks.

The $43.85 billion loss in market value since Jan. 14 comes as investors, surprised by a $1.8 billion charge the company announced Monday, fret that AIG won't be able to sustain its past growth rate.

Perceptions of an AIG with bullet-proof financial strength appear to be changing. These days, the insurer is increasingly viewed as tied to the industry's challenges.

AIG is one of nine publicly traded U.S. companies with the top "AAA" credit rating. The bond rating agencies this week reaffirmed that rating but placed AIG on watch for possible downgrade, a move that could raise borrowing costs and a lower gold-standard reputation.

"AIG is a great company, an insurance industry leader," said Michael Barry, credit analyst at Fitch Ratings, which put AIG on notice. "But because they are part of the insurance market, they are not immune from market conditions."

Companies like AIG face the dilemma of drawing policies with competitive prices while building the reserve they need to protect from losses. Analysts say years of cheap policies have undercut this dynamic during a time of rising liability claims against the companies AIG insures.

Moody's Investors Service also lowered its outlook on AIG's rating twenty years after handing the company a "AAA" rating.

Standard & Poor's, the other major credit rating agency, has not altered its views on AIG.

"This is a bit of a tempest in a teapot," said Jay Dhru, credit analyst at S&P.

"There's no huge smoking gun here," said Alan Murray, of Moody's, which still rates the company's debt "AAA." "It's just a question of incremental pressure."

Those pressures grew when AIG late Monday said it would take a $1.8 billion charge to increase reserves to pay liability claims. The company blamed a rise in lawsuits, a surprise for an insurer known for managing risks to optimize profits.

"That was the first time in recent memory that their estimation tools have not captured what is the current amount of risk in these businesses," Murray said.

Friday, AIG shares fell $1.50 to $46.70, bringing their post-Jan. 14 loss to 26.4 percent. It was AIG's lowest close since Dec., 14, 1998.

The entire industry has suffered of late, with the Dow Jones insurance index off 17.2 percent in the period. Most stocks have also fallen. The Standard & Poor's index has slipped 11 percent since Jan. 14.

Few companies have enjoyed a better long-term run than AIG. Adjusting for 14 stock splits, shares of AIG are up 17,861 percent since the company went public in June 1969, far outperforming the Standard & Poor's 500, which rose 735 percent during those 34 years.

The company's fourth-quarter profits, due next week, reflect Monday's warning. Analysts surveyed by First Call expect that December quarter profits fell to 21 cents per share from 77 cents in the year ago quarter. AIG has been growing annual profits since at least 1996, something analysts expect ended last year.

AIG's $1.8 billion reserve increase equals an average quarter's earnings, said Dhru, who said some hit was to be expected from lawsuits and years of pricing pressures.

Insurers invest billions of dollars in the bond market, where blowups from companies like WorldCom and Enron have hurt returns. The industry also lost money after the Sept.11 terrorist attacks. But AIG has said it faces few problems with the asbestos claims that have hurt rivals.

The last "AAA" company to lose that rating was Bristol-Myers Squibb. The drugmaker left an elite group of firms that in addition to AIG include UPS, General Electric, Merck, Pfizer, Johnson & Johnson, Automatic Data Processing, Berkshire Hathaway and Exxon Mobil.

Maurice Greenberg

No company in that group has a longer-tenured CEO than AIG. Maurice "Hank" Greenberg, 77, has led the company since 1967, prompting questions about when he will retire and who will replace him.

That said, Greenberg, only the second CEO in AIG's history, will likely once again lead next week's earnings conference call with analysts. The company reports results before the market opens Thursday.

"We've heard the bad news," said Ira Zuckerman, who covers AIG for Nutmeg Securities. "Now we have to hear the good news."