Average 401(k) balances, employee contributions and matching contributions from employers all reached pre-recession highs at the end of the third quarter this year, according to a report Thursday from Fidelity Investments, the nation’s largest 401(k) administrator.

In the three months that ended in September, the average balance of 12 million accounts administered by Fidelity grew to $75,900, up 4.2 percent from the end of the previous quarter and 18 percent from the year-ago average of $64,3003.

About three-quarters of this year’s increase was due to a rebound in stock prices, Fidelity said. But a post-election downturn has driven the Dow Jones industrial average and Standard & Poor’s 500 index to their lowest levels in months. The Dow continued to slide Thursday by more than 121 points, and the S&P dropped 17.02 points to its lowest level since Aug. 2.

Concerns that newly reelected President Barack Obama and Congressional Republicans will not reach an agreement to stop large tax hikes and government spending cuts that kick in Jan. 1 are keeping investors at bay, said Curvin Miller, a financial planner at Russell & Co. in Fairborn.

“Everybody has been kicking this can down the road until we knew who was going to be president,” Miller said. “Now that we know, the fiscal cliff seems a little more real and the markets realize there may not be extensions (of Bush-era tax cuts) going forward.”

The long-term capital gains tax on investment income would revert back to 20 percent from 15 percent if the tax cuts are allowed to expire, and many investors are likely selling their stocks to shield themselves from a possible tax hike, Miller said.

Still, most workers are likely to continue to invest in their 401(k) plans because they have few other options. Fidelity reported that average annual employee contributions reached $5,900 in the third quarter, up more than 7 percent from the same period before the recession began five years ago.

“The days of relying on defined benefit plans or pensions is an old way of thinking, and people are waking up to the idea that if they don’t put money into a 401(k) they could be in trouble later on,” Miller said.