"We've confidentially submitted an S-1 to the SEC for a planned IPO," the tweet said. "This tweet does not constitute an offer of any securities for sale."
An S-1 form is a financial document that U.S. companies file with the Securities and Exchange Commission in New York as part of the formal IPO process.
Under legislation passed last year called the Jumpstart Our Business Startups, or JOBS Act, U.S. companies with revenue of less than $1 billion US a year can file a secret IPO and need not make the filing public until 21 days before what is known as the "road show" portion of the IPO process, which the company uses to drum up interest in its stock among investors and fund managers.
Analysts who had been watching the San Francisco-based company ramp up its efforts to boost advertising revenue had been expecting the IPO to be filed some time in October or November.
They will be closely watching Twitter to see if it avoids the pitfalls that plagued Facebook's bungled IPO in May 2012, which many saw as ill-timed. Facebook initially priced its stock at $38 a share but saw its share price plunge after computer glitches on the Nasdaq delayed trading. Its stock price only recovered its value this past July when it traded above $38 for the first time since the IPO.