The former `Sabrina the Teenage Witch` star - who turns 36 on 18.04.12 - announced the news on twitter by posting a picture of her visibly pregnant self along with sons Mason, six, Braydon, four, and husband Mark Wilkerson.
She tweeted: "(Early) Happy Birthday to me! And baby makes 3!"
A source told Life & Style magazine the `Melissa and Joey` star could not be happier about the latest addition to the clan.
The insider said: "She`s really excited and is starting to show. You can barely tell, but there`s a teeny-tiny little bump."
Melissa - who married 35-year-old musician Mark in 2003 - had always hoped to have a large family, and has even been looking in to the possibility of adoption.
Following the birth of Braydon, she revealed: "We definitely want to add to our family. We`re interested in adopting as well. We`re just starting to explore that.
"Since I was little, I knew that I wanted to have children. I`m the oldest of eight kids, and I have always liked the idea of a big family."