“There are three of us in this pic,” she captioned the snap, in which she was wearing a black leather jacket over a loose-fitting blush minidress with crystal embellishments.
Mara was soon inundated with congratulatory messages from fans and fellow celebrities.
“American Hustle” star Michael Peña wrote: “congrats!!!”
Mara and Bell began dating in 2015 after starring together in “Fantastic Four,” in which they played Sue Storm and Ben Grimm, respectively.
The couple are already parents to a daughter born in 2019. Bell also shares son Jack, 8, with ex-wife Evan Rachel Wood.
She said at the time that the loss colored her second pregnancy with her daughter, because although she was excited there was also “a fear element.”