Multiple sources confirmed the news about the couple, who married in 2019, to People magazine.
Reps for Pratt, 42, and Schwarzenegger, 32, have not yet responded to Yahoo Entertainment's request for comment.
They are parents to a daughter, Lyla, 1, and the Guardians of the Galaxy actor has a 9-year-old son, Jack, from his marriage to Anna Faris.
As the news broke, Schwarzenegger was doing an Instagram Live with child speech and language experts and those tuned in begged in the comments for Schwarzenegger to confirm the pregnancy. She didn't.
Earlier this week, Schwarzenegger — daughter of Arnold and his estranged wife Maria Shriver — celebrated her 32nd birthday. Pratt posted a gushing tribute to her on social media, calling her "a wonderful wife, mother, step-mother, and life partner. I simply can’t imagine how lost I’d be without you. You’re beautiful, tough as nails, reasonable, deeply thoughtful, extraordinarily smart, you’re a complete boss, eternally driven and you communicate like no other."
That went over better than a different post about her that he shared in November.
Pratt said in July that growing their family was part of their plan.
"I would love to have more, as many as the man upstairs will provide," he said. "We'll do it. I'll probably have to talk to Katherine about that but my plan is, let's go."
Schwarzenegger opened up to Yahoo Life about motherhood in September, saying, "I try to have as much fun with it as possible, and if I had to describe it in a word or two, it would be 'love' and 'fun' — even when we’re going through a teething moment [laughs]. I have the most well-behaved baby, happiest baby ever and I know I’m lucky. I’ve enjoyed every part of being a mom so far."