FORTUNE – Corporate America has seen profits soar for the past several quarters, but the party may not last much longer. After markets close on Tuesday, manufacturing giant Alcoa will kick off a slew of earnings results for the first quarter. Analysts expect overall earnings in the S&P 500 to significantly slow, possibly signaling the beginning of a turning point in the market. Last year, earnings per share in the S&P 500 grew in the range of 8.4% to 19.7%, but several factors could crimp profits this year.
To get a sense of where earnings might fall, watch for these four key indicators.
The Apple indicator
Apple (AAPL) might only be one company in the S&P 500, but lately it's had far more influence on the index than any one company should. At the end of last year, overall earnings for S&P 500 rose about 6% compared to the previous year. If Apple's 116% profit surge was excluded, earnings would have risen only 3%.
For the first quarter, overall earnings are expected to decline 0.1% compared with a year earlier. But it could be worse without another stellar quarter from Apple. According to a report by FactSet released last week, the S&P 500 could see a deeper 1.6% fall if Apple were excluded. Given that the company's profitability weighs heavier in the technology sector, earnings among the S&P 500 tech companies are forecast to grow 3.4%. Without Apple, earnings would actually fall by 4.7%. The tech company is scheduled to release its earnings after markets close on April 24.
Emerging markets
Even as the U.S. struggles with a tepid recovery, America's biggest companies have seen profits soar. Not only is it largely because executives trimmed costs and laid off workers, but it's also because they've turned to places like India, China and Brazil for sales.
Since the global recession of 2009, the developing world has largely driven growth across the globe while Europe and the U.S. struggle to recover from the financial crisis. But signs that growth in emerging markets is slowing will likely hurt profits at U.S. companies.
Take Alcoa (AA) for instance. Wall Street expects the world's largest producer of aluminum to post a loss as the pace of growth slows in key markets like China and Europe. Indeed, analysts and strategists will be looking at how the company is coping with a glut of inventory and weaker prices, but, as Business Insider has pointed out, they'll also focus on the economic outlook of China – the world's biggest consumer of aluminum.
Fuel costs
Sales may be robust, but paying higher fuel costs to do everything from transporting materials to running machines at factories is expected to erode profits at many big companies.
The average weekly price for Brent oil was $118.86 at the beginning of this year – markedly higher than the average $106.27 recorded around the same period last year.
FactSet senior earnings analyst John Butters expects that 104 companies in the S&P will see healthy growth in sales during the first quarter, but also a decline in earnings. Excluding banks and other financial companies, net profit margins is expected to come in at 8.4%, the lowest since the first quarter of 2010.
Butters adds that consumer-focused companies, such as Nike (NKE), General Mills (GIS), maker of Cheerios and Yoplait yogurt, as well as Carnival (CCL) cruise lines, are especially vulnerable.
And while the U.S. airline industry managed to turn profits amid soaring fuel prices last year, raising ticket prices and fees might not help cushion margins this year. Fuel costs could cause profits across the world's airline industry to plunge 62%, according to the International Air Transport Association's March report. That's a bigger drop than the trade group predicted five months ago. It forecasts that carriers' net profit could fall to $3 billion in 2012 from $7.9 billion last year.
The power of the euro
In recent months, Europe's ongoing debt crisis has helped strengthen the U.S. dollar against a weakening euro. At the start of this year, the euro on average weakened to about $1.31, compared with $1.37 around the same period last year.
Because several U.S. companies derive a majority of annual revenues from international markets, swings in exchange rates can translate to millions of dollars in earnings. And for companies with big sales in Europe, a stronger greenback relative to the euro could certainly slow earnings.
Take McDonald's (MCD). It rode out the recession and the months following almost unscathed. But headwinds are on the horizon. Last year, the Oak Brook-based restaurant chain generated $10.9 billion or 40% of its revenue in Europe. Foreign exchange boosted its earnings by 19 cents per share, but that will likely work against the company this year. During a call with investors in January, the company warned that foreign currency conversion, primarily driven by the euro, could drag down per-share earnings by 16 to 18 cents in 2011.
McDonald's is expected to report earnings on April 20.
Nin-Hai Tseng covers economics and finance. Before joining Fortune, Tseng was a reporter at The Orlando Sentinel and a public affairs associate at GE. She holds an MPA from Columbia University and a BS in Journalism from the University of Florida. She lives in New York City.
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