Fox will also appear alongside Westfeldt's partner and Mad Men star Jon Hamm in the ensemble comedy as well as Adam Scott and Whip It! actress Kristen Wiig.
The plot follows a number of couples in their 30s who reflect on the influence having children has on other people's relationships and resolve to avoid these pitfalls.
"It's a very literate script and when Jen met with her she thought Megan was spot on," The Hollywood Reporter quotes Cinetic Media's John Sloss as saying.
Westfeldt, who wrote the script, is to star in and produce the movie.
Friends With Kids is currently scheduled to be released in April 2011.