Isabella Louise was born a month premature Friday and weighed 5 pounds, 4 ounces. However, she was home in Los Angeles with her parents and siblings Alexandra, 8, and Elizabeth, 2, after only one night in the hospital.
"Mom and baby are fine and everyone is very happy," Astin's rep, David Lust, told People.
Astin -- the son of Patty Duke and John Astin -- played the hobbit Samwise Gangee in the "Lord of the Rings "trilogy. He is about to begin production on Adam Sandler's new comedy, "Click," opposite Kate Beckinsale.