The baby will join big sister, Olive, six, and Elula, who turns five this month.
The 38-year-old Wedding Crashers actress has said that her children occupy her thoughts 24/7.
"Even when you don't take your kids with you, your mind and heart are busy with them," she told Gotham magazine last summer.
"Family is 100 percent my top priority. So I'm much more comfortable doing supporting roles right now. And often," she told The Telegraph in July 2013.
Fisher has said she does not regret choosing motherhood over her career after the birth of her daughters.
— Yahoo Celebrity (@YahooCelebrity) October 1, 2014
"I took three years off," she said. "It's not like you're taking a break and looking on IMDbPro to see your StarMeter falling. You're doing the most important, incredible thing. When you come back in, the perspective has changed. I truly believe you can't have it all and you shouldn't want to."
— HELLO! Canada (@HelloCanada) October 1, 2014
"It's a myth that you can have it all. You can't. But more importantly, I don't think you should want to. It sounds exhausting!"
— E! Online (@eonline) October 1, 2014
The star doesn't sugarcoat the difficulties that arise when trying to balance life as a mom and an actress.
"I'm just dipping my toe back into work now. It's a juggling act and I'm not particularly good at it; the balls are in the air," she said. "But my priority is always my family. Motherhood boils everything down to the essentials."