Melissa McCarthy, one of the breakout stars of Bridesmaids, and Ryan Lee, the child pyromanic in Super 8, are joining the cast of Judd Apatow's untitled Knocked Up spin-off.

Rumored to be eventually called This Is Forty, it stars Paul Rudd and Leslie Mann reprising their roles from the comedy classic and, in the film, McCarthy will play a mother at the school where their kids attend. Lee will play her son.

The movie, which has just begun shooting, will chronicle more hilarious issues with the couple's marriage. Many of the character details are being kept hush-hush, but vague references have leaked over the course of the film's development.

Albert Brooks, Chris O'Dowd, Megan Fox, Lena Dunham, and Judd and Leslie's real-life progeny Maude and Iris Apatow are all on-board to take part in the fun.

Whatever this film ends up being called, it has been scheduled for release on December 21, 2012.

[via The Hollywood Reporter]