The probability the Fed will keep rates at the current 225-250 basis-point level was 98.7 percent on Wednesday, according to CME Group's FedWatch tool. Weak data published this month, including February jobs data that drastically missed expectations, has supported the Fed's pause in rate hikes.

"The realities of the mounting evidence pointing toward a pivot in the global growth cycle offer a troubling backdrop for today's Fed decision," said Ian Lyngen, head of US rates strategy at BMO Capital Markets.

Among recent revelations of slowing growth, he cited profit warnings from major European companies including German automaker BMW and Swiss bank UBS Group as well as the geopolitical headwinds of Brexit and US-China trade negotiations.

Investors were also concerned about FedEx Corp's, 2019 profit forecast which the company cut because of "softening macroeconomic conditions." The package delivery industry is widely seen as a bellwether for the global economy and FedEx's drop in share price helped pull the Dow Jones Industrial Average down 1.4 percent on Wednesday morning.

In January, the Fed pivoted after hiking rates four times in 2018, pledging patience before making further moves. In February, Fed Chair Jerome Powell also said the central bank would stop shrinking its balance sheet later this year.

Although unchanged rates have been priced in, investors will be watching to see if the Fed's dot plot, the diagram which shows individual committee members' rate views for the coming three years, aligns with the patient approach the Fed has expressed. Investors will also be looking for details on whether the central bank will continue to shed bond holdings from its balance sheet.

"In pondering the order of importance, or at least price action relevance, for the potential new information the number one slot undoubtedly goes to the 2019 dots, trailed closely by the balance sheet clarity," said Lyngen.

The 10-year Treasury yield was last at 2.589 percent, down 2.3 basis points. The two-year Treasury yield , a proxy for investor expectations of interest-rate hikes, was down 1.3 basis points, at 2.456 percent. At the other end of the maturity range, the 30-year yield was up 2 basis points at 3.006 percent.