Central banks rebuilding foreign-exchange reserves at the fastest pace since 2004 are crowding out private investors seeking U.S. dollars, boosting demand even as the Federal Reserve considers printing more currency.

After falling to an all-time low of 60.5 percent in the second quarter of last year, the dollar's share of global reserves rose 1.6 percentage points to 62.1 percent in December, the latest International Monetary Fund figures show. The buying has left the private sector with $2 trillion less than it needs, according to investment-flow data by Morgan Stanley, which sees the dollar gaining 8.2 percent in 2012, the most in seven years.

While the Fed has created more than $2 trillion under its stimulus programs since 2008, the flows signal that there may actually be a shortage of dollars to meet demand as Europe's debt crisis deepens and the global economy slows. The dollar has risen 3.5 percent since the end of April against a basket of the most-widely traded currencies even amid speculation that the Fed, which meets this week, may undertake the type of stimulus measures that weakened it in the past.