"I can't talk about the pregnancy too much," Howard said amid congratulations. "I'm a little superstitious."
When asked about his costar's condition, Lady leading man Paul Giamatti replied jokingly, "I had nothing to do with it!"
Howard, 25, and Gabel, 24, wed in June after five years together.
Howard got early acting experience in 1989's Parenthood and 1995's Apollo 13, both directed by her father, actor-filmmaker Ron Howard.
Her breakout role was in The Village in 2004, and she will next be seen in Spider-Man 3.