About 15 minutes into trading, the Dow Jones Industrial Average stood at 24,067.04, down 0.1 percent.
The broad-based S&P 500 had shed 0.3 percent to 2,648.24, while the tech-rich Nasdaq Composite Index lost 0.1 percent at 7,123.92.
Analysts expect the Fed to keep interest rates unchanged but the US central bank could signal a likely rate hike at its June meeting. The prospect of further Fed tightening has been among the top worries to pressure stocks this year.
US private employers added 204,000 employees last month, a bit below analyst expectations, according to payrolls firm ADP. The more closely-watched US government report on Friday will also be assessed in terms of what it means for Fed policy.
Apple jumped 3.1 percent after reporting better-than-expected earnings late Tuesday and unveiling a new $100 billion share buyback plan. The results helped ease worries over prospects of the world's biggest company by market capitalization.
Gilead Sciences plunged 8.1 percent after reporting a 43.1 percent drop in first-quarter profit to $1.5 billion following a big drop in sales for hepatitis drugs.