The 43-year-old actress and 50-year-old actor revealed as much in a video published Monday on E! News after announcing Spelling's pregnancy in October.
"Well, it's a tie-breaker -- boys win!" the pregnant star said at her family's reveal party. "We're just going to have one more then."
Spelling and McDermott married in 2006, and are already parents to 9-year-old son Liam, 8-year-old daughter Stella, 4-year-old daughter Hattie and 4-year-old son Finn. The actress had admitted in the Oct. 17 issue of People that her latest pregnancy was "a total surprise."
"Dean was saying, 'We just got Finn out of diapers; I thought we were in the clear!'" she said. "For the first time this year, they're all in school. So it's like, wow. We're basically starting over."
"This baby happened at the best time," the star qualified. "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."
Spelling told E! News the same month that she and McDermott have yet to pick a name for their unborn child. She also said it's "really nice" to experience a pregnancy while her kids are older.
"I had them all pretty quickly and all back to back, so they were little," the actress explained. "Now they're four through nine, so they can enjoy this pregnancy and experience it with me. It's a whole different experience."
Spelling is known for playing Donna Martin on Beverly Hills, 90210, and has since appeared in a number of TV movies. She and McDermott also starred on the Lifetime reality series True Tori.