Montag, who met her husband on the set of The Hills, said she broke down in tears on finding out the news.
Speaking to US Weekly, Pratt said: “The look on her face, I can’t even describe it. She was literally glowing. I thought she was about to say she made muffins or banana bread.
“Heidi said, ‘I’m pregnant’. I was like, ‘Whoa, that’s way more exciting than banana bread’.”
Montag – who is due on October 19 – said: “I started crying, and he embraced me.
“I have never been more excited. The reality is sinking in that we are going to have a child! I’ve read every pregnancy book, and now I realise I don’t know anything.”
The couple, who have been married for eight years, said they decided to wait until now to try for a child as they had too many work commitments, including Celebrity Big Brother.
Montag said she has been suffering from morning sickness, but that knowing she will soon be a mother is helping her stay “euphoric”.
She said: “I’ve had really bad morning sickness since the beginning, but I’ve never been so happy to feel so sick!
“Every time I feel awful, I remember why and I’m just euphoric. It’s so fulfilling, and I’m mostly just grateful. I’ve been with the love of my life for 10 years, and now we’re starting our own family.”