The 7th Heaven star and her husband, Michael Cameron, are set to welcome a baby in April (13).
Speaking about her pregnancy to People.com, Mitchell reveals she became aware of the impending arrival when her sense of smell became heightened four months ago.
She says, "My sense of smell was off the charts, to the point where I could smell a kid's dirty diaper from the other side of a restaurant.
"I was like, 'No, it can't be that'. I took seven pregnancy tests. I went to the doctor and had two more tests and they were like, 'Bev, usually when the first one comes out positive, the rest of them will all say the same thing.'"