The star is due to give birth in the new year and is already a proud mum to five-year-old daughter Ramona with husband, Peter Sarsgaard.
The 34-year-old actress, who starred in Batman hit, The Dark Knight, confirmed the news to People magazine in the States, after the rumour mill went into overdrive over her rounded tummy, which suggested she was quite far along in her pregnancy.
Maggie married Peter in 2009, and has been open about her struggles with first time motherhood.
"I've spent all my time learning how to be a mother and make a home with a child. When Ramona was first born, I didn't know what I was doing," admitted Maggie.
"I just didn't have a clue. There's no way to prepare for the challenges, the immense joys, the surprises, the disappointments and the shocks. Your heart just rips open. It's amazing."
Congratulations Maggie and Peter!
See who else is expecting this year in our gallery of pregnant famous faces...