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The 26-year-old confirmed that she is expecting her first child with the film producer earlier this month.
She has been dating Warren since he worked as a director's assistant on the set of her 2004 movie Fantastic Four, although they separated briefly during the summer.
Her publicist Brad Cafarelli announced the engagement after the actress was spotted wearing a diamond ring over the Christmas break.
He told the Associated Press in an email: "I can confirm that they are engaged."
A source close to the couple recently claimed that Alba does not want to get married while she is pregnant.
