Christina, 33, and film producer Matt, 29, are set to welcome their first child together. "They're very much in love and are really excited to take this next step," a source close to the pair told People.
The Voice coach is already a mum to Max Liron, six, with her ex-husband Jordan Bratman, to who she was married from November 2005 to October 2010.
Christina and Matt have been together almost three years. They first met on the set of her 2010 film Burlesque.
On Valentine's Day, Christina announced their engagement via Twitter. "He asked and I said….," she wrote alongside a sweet photo of her diamond-adorned hand holding Matt's in front of a romantic beach setting.
The happy couple have been living together in the Grammy-winning vocalist's new Mulholland Estates mansion in LA, which she bought in April 2013 for $10 million.