The cheeky "Iron Man" star said "I think I will be wrist-deep in doo-doo within 18 months, I'm calling it, right here."
Downey Jr already have a 17-year-old son from his first marriage with actress Deborah Falconer. During his interview with Playboy he admitted that he would like to have children with his second wife Susan.
"I think we should probably try to have a girl because I don't want another male entity to have to compete.
"I don't want Indio to feel there's another boy in my life," said Downey Jr.