Hartnett, 37, and 27-year-old Egerton were seen leaving St Mary's Hospital in London on December 1, with the proud father carrying the new arrival in a car seat, reported Female First.
The couple have not yet confirmed whether they had a boy or a girl, or any other details about the baby's birth.
The pair, who first met on the set of "The Lovers" in 2011, confirmed in July they were expecting a baby.
Their representative said at the time: "It's true and they are beyond thrilled."
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