New York — Stocks sank sharply Monday, with the S&P 500 falling to its lowest level in more than a year and the Dow shedding 508 points amid ongoing concerns about U.S. economic growth. The S&P 500 index is down more than 2 percent in 2018 and has fallen nearly 13 percent since setting a record high in late September.

The Dow and Nasdaq each fell more than 2 percent on the day and are both in the red in 2018. The Russell 2000 index of small-cap stocks is down 20 percent since it finished at its last record high at the end of August. Wall Street calls a 20 percent decline a "bear market," and it's considered a major downturn.

The S&P 500 lost 54 points to close at 2,546, a 14-month low.

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"Absent an extreme Christmas rally, most portfolios are going to show somewhat negative performance in 2018," said UBS analyst Michael Crook, in a report.

Obamacare ruling hits health stocks

Shares of health insurers and hospitals slumped after a federal judge in Texas ruled that the 2010 Affordable Care Act is unconstitutional. Hospital operator HCA dropped 2.5, while health insurer UnitedHealth lost 1.9 percent. Centene, a health insurer that focuses on Medicaid and the Affordable Care Act's individual health insurance exchanges, also lost ground.

Many experts expect the ruling will be overturned, but with the markets suffering steep declines in recent months, investors didn't appear willing to wait and see.

Retailers and technology stocks also sank. Some of the biggest losses are going to utilities and real estate companies, which have done better than the rest of the market during the turbulence of the last three months.

Fed set to hike interest rates?

The Federal Reserve is expected to raise interest rates again Wednesday, the fourth increase of this year. It's been raising rates since over the last three years, and investors will want to know if the Fed is scaling back its plans for further increases based on the turmoil in the stock market over the last few months and mounting evidence that world economic growth is slowing down.

President Donald Trump, who has repeatedly complained about the Fed moving to hike rates, renewed the attack Monday. He said in a tweet: "It is incredible that with a very strong dollar and virtually no inflation, the outside world blowing up around us, Paris is burning and China way down, the Fed is even considering yet another interest rate hike. Take the Victory!"

Many economists and market analysts expect the Fed to slow the pace of interest rate increases next year to reflect what most forecasters say is likely to be slower U.S. growth.

"Our expectation is for two interest rate hikes in 2019, and we think this week the Fed will open the door to less aggressive fed fund increases, which would bring the Fed in line with our expectations," analysts with Keefe, Bruyette & Woods said in a client note.

Trade tensions flare

China and the United States clashed again over their respective trade policies Monday, as China criticized what it calls a "unilateralist and protectionist" approach to trade. The U.S. ambassador to the World Trade Organization said those critiques were unwarranted.

The two nations have been embroiled in a dispute over technology policy and other issues for most of this year. With no end to the conflict in sight, investors are growing more concerned that the tensions will drag down the already-slowing global economy.