The reality star couple announced in the April 24 issue of Us Weekly that Montag, 30, is pregnant, expecting a baby in October.
"I have never been more excited," Montag told the magazine. "The reality is sinking in that we are going to have a child! I've read every pregnancy book, and now I realize I don't know anything."
"Every time I feel [sick], I remember why and I'm just euphoric," she added. "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."
Montag came to fame on the MTV series Laguna Beach before starring with Pratt on The Hills. The couple married in 2009, and have since appeared on Celebrity Big Brother, Celebrity Wife Swap and Marriage Boot Camp.
"I wanted a baby three years ago," Montag said. "Spencer was a little hesitant. Then one thing after another came up. We had work obligations, like Celebrity Big Brother in the U.K., which I did not want to be pregnant for. I'm actually really thankful we waited."
"It might never be the perfect time, so we said, 'Let's move forward.' We had all these conversations. We're older, we have more life experience. Nothing is holding us back," she said.
Montag and Pratt's former Hills co-stars Lauren Conrad and Whitney Port are also expecting. Their Celebrity Big Brother Season 19 co-star Bianca Gascoigne was among those to offer their congratulations online.
"Congratulations!! @heidimontag & @spencerpratt your going to be amazing parents," the British television personality tweeted Wednesday.
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