Robert Downey and his wife, producer Susan Downey, are expecting a baby boy.
The actor broke the news Monday night on NBC's The Tonight Show With Jay Leno, where he was promoting the upcoming Sherlock Holmes sequel.
He told Leno that he wasn't allowed to reveal the baby's gender, but then blurted it out.
"I am not permitted to discuss it. I can't say a word," he said, adding "We're having a boy!"
He then put his head in his hands and feigned horror, saying "S---. I did it."
As for how the pregnancy has been going, Downey joked that "it's been tougher on me. Just the hormones and the moods stuff and the nausea, and the whole thing."
The Downeys announced their pregnancy in August; this will be the couple's first child together. The actor already has a son, 18-year-old Indio, with ex-wife Deborah Falconer.
Sherlock Holmes: A Game of Shadows opens Dec. 16; Susan Downey is one of the film's producers.
Downey also reprises his role of Iron Man for The Avengers, which bows May 4.