Analysts say the risks have risen for a slowdown in US growth that might persuade the Fed to hold off from a rate hike for a few more months.
Global markets remained cautious ahead of the decision, to be released at 1800 GMT Thursday. There were gains in most major stock markets Wednesday, but only slight shifts in the major currencies.
The decision, though, has been preceded by calls for the US central bank to move gingerly.
The World Bank has warned developing economies to prepare for more capital and currency market turmoil while the OECD urged the Fed to move slowly and make its policy plans clear, whatever it decides.