Robert Downey Jr. has joked that he wants to call his baby boy "I am Forbidden".

The actor is expecting a son in February with his wife, Susan Downey.

On Monday the actor let slip the gender of the baby during an interview with Jay Leno and has now spilled even more details.

When asked about his favourite names at the premiere for Sherlock Holmes: A Game of Shadows, the star told ET he had a preference for "I am Forbidden".

Susan, who is also a producer on the movie, looked horrified at the Robert's suggestion and quickly laughed it off.

The 46-year-old also claims he has been suffering during the pregnancy.

"This trimester has been really tough for me," he said with a straight face.

Susan agreed and said her husband of six years had recently been experiencing mood swings and an upset stomach.

"He's pregnant and I'm carrying it," she laughed.

Robert has an 18-year-old son, Indio Falconer, from his previous marriage to Deborah Falconer.

The star is currently promoting his new movie Sherlock Holmes: A Game of Shadows. He has reprised his role as the title super sleuth, while Jude Law plays his sidekick Dr. Watson.