The A Lot Like Love actress, who is has daughters Frances Pen, seven, and Molly June, four, with her novelist husband David Benioff, revealed the news to People magazine.
Amanda is pregnant with her third child
"I'm very excited," Amanda told the publication while supporting the UN Foundation and Walgreens's Get a Shot, Give a Shot campaign in Los Angeles. "[My daughters] are very excited now. We'll see what happens when he actually arrives.
"I'm definitely very tired," she added. "I'm 42 and it's my third baby."
Amanda revealed the news at the Get a Shot. Give a Shot. campaign in LA with Jo Frost and Ali Nejad
When quizzed on whether she has prepared her home for the little one's arrival, Amanda admitted that she "hasn't given a lot of thought" to arranging a nursery as she has been concentrating on preparing her daughters for having a baby brother.
"We talk about it," she said. "They talk to him and listen to him."