The actress, who will be starring in the upcoming Gone Girl, confirmed on Thursday to E! News that her and partner Robie Uniacke will be parents again.
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The couple already have a two-year-old son, called Solo.
The 35-year-old actress has been with partner Robbie, 53, since 2009. Robie already has four children from a previous relationship.
Perhaps best known for her role as a Bond girl in Die Another Day, Rosamund has previously spoken of her family life, telling Vogue magazine: "It is interesting to break all the rules. I'm not married, I have a baby, and it feels infinitely more right."
Rosamund Pike and Robie Uniacke
The blonde beauty beat out many high profile actresses for the role of Amy Dunne, a woman who is reported missing by her husband Nick, played by Ben Affleck.
Nick however soon becomes the main suspect in the investigation into her disappearance but the plot continues to twist and turn.
Based on Gillian Flynn's 2012 novel, which topped the New York Times bestseller list for eight weeks and sold more than two million copies in its first year of publication, fans have been eagerly awaiting their chance to see Ben Affleck and Rosamund in action in the adaptation.
Gone Girl is released in the UK on 3 October.