The 24-year-old Duff says in an interview on "The Ellen DeGeneres Show" to air Monday that as soon as the pregnancy test turned positive, she knew she had a son on the way.
"I had a feeling it was a boy the whole time," Duff said.
The baby is due early next year and is the first for Duff and husband Mike Comrie, People magazine reported.
This also is Duff's first pregnancy after a number of false alarms and has been going relatively well so far. "No sickness, but crazy dreams," she said. "It's kind of like a little alien has taken over your body. You just don't quite feel normal."