NEW YORK (Reuters) -U.S. companies could report milder earnings growth in the third quarter than earlier this year, partly due to a likely tariff hit, while investors look for signs that heavy spending on artificial intelligence is paying off.

While most U.S. corporations have managed to beat earnings expectations even after U.S. President Donald Trump first announced wide-ranging tariffs on imports in April, the full impact of his trade policies remains uncertain.

With optimism about emerging AI technology lifting Wall Street indexes to record highs this year, investors are likely to focus more on AI-related capital expenditures than tariffs and other risks.

Analysts expect S&P 500 companies to post 8.8% higher earnings than in the 2024 third quarter, according to the latest LSEG forecast. Year-over-year growth was above 13% in each of the first two quarters of 2025.    

The reporting period unofficially kicks off next week with results from some of the biggest U.S. banks. 

The S&P 500 and Nasdaq registered record closing highs on Wednesday, boosted by AI-related megacaps that have been market leaders this year.

Earnings from the so-called Magnificent 7 group of megacap stocks and AI leaders could be "very robust," said Anthony Saglimbene, chief market strategist at Ameriprise Financial in Troy, Michigan.

"But investors are going to be skeptical about some of the commentary on capex spend," he said. "Investors are starting to get a little more concerned about the money that's being spent there, and the payback for that investment."

Valuations have hit levels some investors consider frothy, with the S&P 500 now trading at roughly 23 times forward earnings estimates and well above its 10-year average of 18.7, based on LSEG Datastream data. High valuations in megacap technology and growth stocks present a particular risk given the worries about AI investment and spending.

Companies continue to pump more money into AI and negotiate AI-related deals, such as AMD's announcement earlier this week that it will supply chips to OpenAI in a multi-year agreement.

Investors are especially keen to see quarterly results for guidance because the U.S. government shutdown that began October 1 has held up official economic reports from the federal government.

The information void has made it more difficult to get a sense of how the economy is doing and how the Federal Reserve might proceed with interest rates. The U.S. central bank last month cut rates for the first time since December amid labor market weakness.

Still, mostly upbeat economic data during the last quarter likely means third-quarter S&P 500 earnings and sales growth will surpass consensus expectations, according to Goldman Sachs' ( GS ) chief U.S. equity strategist, David Kostin, and his team.

Goldman strategists wrote in a recent note that they expect companies to have maintained their profit margins, even as tariffs "were likely a larger headwind to profits in 3Q than they were during 2Q," and said customs duties rose 33% over that period to $93 billion.

Sales likely grew 5.7% for S&P 500 component companies compared with the year-ago quarter, according to LSEG. Sales rose 6.4% in the second quarter and 5% in the first quarter.

An investment banking rebound is expected to have helped push the six largest U.S. banks to stronger third-quarter earnings.

   In the second quarter, S&P 500 earnings grew 13.8%, while analysts at the start of July estimated just 5.8%, LSEG data showed.

"So far neither tariffs nor economic uncertainty by the consumer have had a negative impact" on earnings growth, said Oliver Pursche, senior vice president and advisor for Wealthspire Advisors in Westport, Connecticut.