(Bloomberg) -- A double-digit selloff in UnitedHealth Group Inc. is dragging down the blue-chip Dow Jones Industrial Average, pushing it toward a day of historical underperformance.

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The Dow Jones Industrial Average fell 1.2% Thursday, weighed down by a 18% plunge in UnitedHealth after the firm cut its annual forecast and reported its first earnings miss in more than a decade. The S&P 500 Index gained as much as 0.6%.

It would make for a strikingly rare divergence, if it lasts through the trading day. The only day ever when the 30-member Dow closed 1% lower or more while the S&P 500 gained at least 0.5% was in 1954, data compiled by Bloomberg show.

“Though the major indexes generally move together, today’s move is that rare occasion when a high-priced stock in the Dow Industrials makes an outsized move that creates a divergence,” said Michael O’Rourke, chief market strategist at JonesTrading.

A key difference between the Dow and the broader S&P 500 is the methods used to weight their constituent stocks. The Dow is price-weighted, meaning that changes in the highest-priced stocks have greater impact on the index level than price changes in the lower-priced stocks. The S&P 500, however, is market-cap-weighted.

Health insurer UnitedHealth Group, the top-weighted stock on the Dow, tumbled the most since 1999 after reporting a first-quarter profit that fell short of analysts’ expectations. The company also lowered its earnings outlook for the year. The guidance cut is rare for UnitedHealth, which typically forecasts conservatively and often raises its outlook as the year progresses.

It would be quite the turnaround for the Dow, which has weathered this year’s market turmoil better than its two broader counterparts. As of Wednesday’s close, the Dow has fallen 6.8% this year, compared to the S&P 500’s 10% drop and the tech-heavy Nasdaq 100’s 13% decline.

UnitedHealth is followed by Goldman Sachs Group Inc., Microsoft Corp. and Home Depot Inc. in the Dow’s rankings, while the S&P 500’s top seats go to tech behemoths Apple Inc., Microsoft, Nvidia Corp., Amazon.com Inc. and Meta Platforms Inc.

Meanwhile, the broader stock market edged higher as traders pore through the latest batch of earnings reports, trying to gauge the impact of President Donald Trump’s tariff offensive.