US STOCKS-Wall St gives up gains as GDP raises Fed concerns
* Twitter rallies after blockbuster results
* GDP shows stronger economic growth than expected in Q2
* Biotechs rally, but insurers fall
* Dow down 0.4 pct, S&P down 0.1, Nasdaq up 0.3 pct (Updates to midday trading)
By Ryan Vlastelica
NEW YORK, July 30 (Reuters) - U.S. stocks were flat on Wednesday, paring earlier gains as a strong read on second-quarter economic growth sparked concerns the Federal Reserve may adjust its stimulus earlier than expected.
Major indexes opened higher but lost their gains throughout morning trading. While the Nasdaq remained in positive territory, helped by biotech shares, the Dow was pressured by a drop in insurer UnitedHealth Group Inc.
Gross domestic product grew at a 4 percent annual rate in the second quarter, above the 3 percent rate that had been expected and a sharp reversal from the weather-impacted first quarter, when the economy contracted a revised 2.1 percent.
The data comes ahead of a statement from the Federal Open Market Committee this afternoon, as the Fed concludes its latest policy meeting. The central bank is widely expected to trim monthly asset purchases to $25 billion from $35 billion, and investors are looking for any hint on when officials could start to reverse their monetary accommodation. Continuación...