BERLIN, April 15 (Reuters) - The German economy probably shrank by 1.8% on the quarter in the first three months of this year due to COVID-19 restrictions, leading economic institutes said on Thursday as they revised their joint growth forecast for Europe’s largest economy. The institutes now expect gross domestic product to grow by 3.7% this year, sharply down from their previous forecast of 4.7%. But the institutes revised upwardly their GDP estimate for 2022 to 3.9% from 2.7% as they expect household spending to bounce back once coronavirus restrictions are lifted again. (Reporting by Michael Nienaber Editing by Madeline Chambers)