Montag revealed the news in Us Weekly's May 22 issue, on stands now.
"It was in my heart to have a boy," the former star of MTV's The Hills said. "I thought Spencer really used to want a boy, and then a few months ago we had a conversation where he said, 'Let's just make sure it's healthy. Boy or girl, I don't care.'"
The reality tv star shared that she started screaming when she heard the news. She also revealed that Pratt is very excited to have a "mini-me."
"I hope to have a little girl one day, but for my first child, I really wanted a son. I'm hoping to God that we don't just have one, but if we do, I wanted a son. I thought it'd be a more natural bond for Spencer since he and his dad are so close," Montag said.
As for what the little guy will be into, Montag said she can't wait to give him all the experiences and let him decide.
"I'm excited to take him to Disneyland and to see if he's into dinosaurs, Mickey Mouse or ballerinas—it can be anything," the MTV star told Us Weekly. "Maybe he'll be the best dancer!"
This whole pregnancy thing is new to Montag, but her fellow Hills co-stars have been helping her out. She has gone to Audrina Patridge and Kristin Cavallari both asking for their best tips.
"I obviously asked both of them for advice," Montag said. "Kristin sent me a whole list of baby things you need in the first few months, which has been really helpful."
Montag also revealed that their son is due in October! We can't wait to see the little one when he arrives!