S&P/Case-Shiller index down 18.1% year over year, but monthly drop narrows to 0.6% in April.

NEW YORK (CNNMoney.com) — Home prices continued to tumble in April, falling 18.1% from a year earlier — but the change from March narrowed sharply, indicating that housing markets may be starting to turn.

The 20-city slice of the S&P/Case-Shiller Home Price index recorded a drop of 0.6% from March to April, compared with a 2.2% drop in the prior month. The index has declined every month since July 2006.

“The pace of decline in residential real estate slowed in April,” says David Blitzer, Chairman of the Index Committee at Standard & Poor’s. “Thirteen of the 20 metro areas also saw improvement in their annual return compared to that of March.”

Not only that but every metro area save one — Charlotte, N.C. — reported improvement in their monthly return compared with March.

“While one month’s data cannot determine if a turnaround has begun, it seems that some stabilization may be appearing in some of the regions,” said Blitzer. “We are entering the seasonally strong period in the housing market, so it will take some time to determine if a recovery is really here.”

Blitzer pointed to some factors that may be lifting the housing markets. For one thing, the stock market bottomed out in March and started a strong recovery. The S&P 500 has gained about 37% since then. Consumer confidence has also improved, making house hunters more likely to pull the trigger on deals.

Not all optimistic: The housing market picture is still very murky, according to Pat Newport, a real estate analyst with IHS Global Insight. He’s not convinced that the improved April report means much more than a seasonal variation in housing markets. Spring is, historically, a strong time of year for housing markets.

He said that not only are home prices still falling but other metrics, such as unemployment and foreclosure rates, are worsening as well.

“Foreclosures are still driving markets, and the rate of foreclosure is still going up,” Newport said. “I think that’s going to continue”

Job losses will all but guarantee that will happen, according to Newport, especially since price declines have put so many homeowners underwater, owing more on their mortgages than their homes are worth. By some calculations as many as 20% of homeowners are underwater.

When people are underwater and they’re losing their jobs or some of their income, that’s bound to result in more foreclosures, more vacant homes for sale and more downward pressure on prices.

Huge declines from peaks: Phoenix, where homes have lost 35.3% of their value over the past 12 months, was the worst performing market over that period. Las Vegas prices plunged 32.2% and San Francisco dropped 28%.

Denver prices fell the least over the last 12 months, down 4.9%, followed by Dallas at 5% and Boston at 7.7%.

Prices in Dallas rose 1.7% between March and April, the largest increase among the 20 cities. Las Vegas prices dropped 3.5%, the biggest decline — which was still narrower than the month before.

Dallas also has suffered the smallest decline from the top of its market, off just 9.6% from its peak in June 2007. The rest of the cities have all suffered double-digit percentage drops from their peaks, with the worst being Phoenix, down 54.1% from June 2006.