"It was a total surprise," Spelling, 43, said. "But we always wanted a big family. I’m really excited."
The couple are already parents to Liam, 9, Stella, 8, Hattie, 5 and Finn, 4. McDermott also has an 18-year-old son, Jack, from a previous marriage.
"Dean was saying, 'We just got Finn out of diapers; I thought we were in the clear!'" Spelling said with a laugh. "For the first time this year, they’re all in school. So it’s like, wow. We’re basically starting over."
Joked McDermott, 49, "We need a Sprinter van!"
For the reality star couple, who recently celebrated their 10-year anniversary after a rough patch, baby No. 5 is a welcome addition.
"This baby happened at the best time," Spelling said. "Nothing is ever perfect, but I’m so madly in love with my husband and with our kids. The idea of adding to that is such a blessing."