The former "House of Cards" actress was first hit with baby gossip in January, when editors at New York Post's Page Six reported that she had told Emily Blunt her happy news at the Golden Globes, where she had worn a billowing dress.
Kate never publicly addressed the speculation, but confirmed she was indeed expecting her first child with husband and "The Fantastic Four" co-star Jamie Bell by showing off her bump in a white and silver Giambattista Valli dress at Elton's John's annual AIDS Foundation Oscars Party on Sunday.
She later shared a picture of her and Jamie on the red carpet on Instagram and added in the caption, "Went on a date with our bun in the oven."
Her stylist Johnny Wujek also posted a snap and wrote: "We're having a Baby. My love @katemara rockin @giambattistavalliparis and the cutest baby bump I've ever seen. Oh & Hi Jamie, you handsome devil you. Proud Gunkle #stylistLife."
Kate, 35, and Jamie, 32, began dating in late 2015 after meeting on the set of the superhero movie flop and surprised fans and followers by tying the knot in July 2017.
The baby will be Bell's second child - he has a five-year-old son from his marriage to actress Evan Rachel Wood, which ended in 2014.
The couple are private about their relationship, but Kate, who was reported to be five months along in January, opened up about their relationship in an interview with Shape magazine last year.
"Now that Jamie is legally part of my family, it's such an incredible thing. I'm very protective of our marriage," she said. "All the travelling we both have to do for work can make it hard, but we have a rule that we will never go longer than two weeks without seeing each other. Even if we have to spend all day travelling to have just 24 hours together, it's worth it in order to have a strong relationship."