The reforms, which aim to strengthen financial resources of the global crisis lender and give more voting power to emerging economies like China, have been endorsed by all the world's economic powers but one.
They were to be put in place two years ago. But although the White House has endorsed them, without ratification by US Congress where mainly Republicans have blocked the measures for political reasons the reforms cannot be implemented.
During the annual IMF-World Bank meetings in Washington on Thursday, a frustrated Lagarde, the 58-year-old former French finance minister, offered up an incentive.
"I will do belly-dancing if that's what it takes to get US to ratify," she said.
The dance will take place "the moment they do."