Banks will play the author of a children's book about breast feeding who is militant about what makes a good mother — until she gets pregnant for the first time.
The movie is based on Heidi Murkoff's bestselling pregnancy manual. Heather Hach wrote it for the screen and Shauna Cross rewrote it.
"What to Expect When You're Expecting" is an ensemble romantic comedy about four interconnected couples.
Murkoff's book has sold more than 16 million copies and has been a New York Times bestseller for more than 500 weeks.
Lionsgate is also banking on Banks as Effie Trinket in its upcoming "The Hunger Games."
The movie begins shooting this summer and is scheduled to be released Mother's Day weekend 2012.