“Went on a date with our bun in the oven,” Mara captioned her photo.
The baby on the way will be the first for the House of Cards alum, 35, while Bell shares a 5½-year-old son with ex-wife Evan Rachel Wood.
Mara first sparked pregnancy rumors at the 2019 Golden Globe Awards, where she rocked a billowing gown featuring a bodice with a cut-out center.
Mara and Bell have been dating since fall 2015, and PEOPLE confirmed their engagement in January 2017. The former Fantastic Four costars announced in July on social media that they had tied the knot.
Mara posted a photo of herself and Bell, 32, kissing on what looks like the dance floor at their wedding on Instagram. She captioned the photo, “Nuptials.”
Bell shared the same photo with the caption, “Mr. and Mrs. B.”
Speaking to James Corden in January 2018, Bell said, “We actually got married right across the street from our house. I literally woke up in my own bed and walked across the street, and we had our ceremony.”