She told her 840,000 followers on Wednesday that the couple were "thrilled" to be expecting a brother or sister for son Kai, who was born in November 2009.
"A few people guessed, so news is out ha!! Me wayne and Kai are thrilled that we are expecting a new addition to our family around May time x," she tweeted.
"Its still only early but bursting with excitement! Also wanted the news to come from us, before anyone else! ??," added the expectant mother.
The childhood sweethearts wed in a lavish ceremony in June 2008 but the relationship twice hit the rocks when newspapers reported that Rooney had paid for sex with prostitutes.