Spelling, 33, and McDermott, 39, have been married since May. McDermott has a son Jack, 8, from a previous marriage.
She tells People magazine, "We are thrilled to be having a son together. Little boys are a ton of fun. We love talking to him. We play tunes for him and watch him kick.
"He already loves the Dixie Chicks! We went to their concert and he kicked the whole time."
McDermott adds, "Now Jack will have the baby brother he’s been waiting for."
Spelling has previously said that if she has a boy he will be given the middle name Aaron after her late father, legendary TV producer Aaron Spelling, who died in June.
They are still deciding on a first name.
She adds, "Yes, his middle name will be Aaron. Dean and I feel it’s the perfect way to honor the memory of my father."