The building society said property values had fallen in three of the last six months and risen in three, because prices were "static".
The average home cost 1.3% less in April than the same month a year ago.
A typical home cost £165,609, and the building society said a strong rebound in prices was "unlikely" in 2011.
Robert Gardner, Nationwide's chief economist, said it was not unusual to see a pattern of modest increases and decreases in house prices when the market was fairly static, as it had been since last summer.
"A strong rebound in the market remains unlikely as the recovery is still expected to remain modest by historic standards," Mr Gardner said.
"In our view, the most likely outcome is that house prices will continue to move sideways or drift modestly lower through 2011."
The three-month on three-month comparison showed that prices rose by 0.6% in the three months to April compared with the previous quarter, Nationwide said.
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