"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," says Mitchell, 31, best known for her role as Lucy on the long-running drama 7th Heaven.
The actress, who plays Kaitlin on The Secret Life of the American Teenager, soon learned the reason for her acute new sense: She and husband Michael Cameron are expecting their first child in early April, she tells PEOPLE exclusively.
Still, the news took a little while to sink in.
"I was like, 'No, it can't be that,'" says Mitchell. "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.'"