The House of Cards actor shared the news with her Instagram followers on Sunday 10 June, posting a photograph of her and Bell holding hands as they left a venue in London.
“There are three of us in this pic,” she wrote in the caption.
The photograph was taken at the end of June when the couple attended the British Film Institute Chair’s Fellowship Dinner at Claridge’s hotel.
Mara concealed her growing bump in a blush pink Miu Miu minidress adorned with pearl and diamante detailing across the neck.
She paired the loose-fitting design with a leather bicker jacket and matching black clutch bag.
Messages of congratulations for the couple have poured in from celebrity friends and fans.
“Congratulations!!!” fellow actor Octavia Spencer wrote.
Michael B Jordan commented with three star emojis, while Jenna Dewan wrote: “Awwww congratulations loves!!!!”
Mara and Bell first began dating in 2015 after starring together in Fantastic Four, in which she played Susan Storm, while Bell played Ben Grimm.
The couple, who maintain privacy around their relationship, became engaged in January 2017 and were married in July later that year.
In May 2019, Mara announced on Instagram that they had welcomed their first child together, a baby girl. The couple are yet to publicly disclose their daughter’s name.
In an appearance on Dr Berlin’s Informed Pregnancy podcast in June 2019, Mara revealed that she had suffered a miscarriage during her first pregnancy.
Mara said she had gone in for a routine scan when the doctors discovered she had a blighted ovum.
Also known as an anembryonic pregnancy, this occurs when a fertilised egg implants in the uterus but does not develop into an embryo.
Opening up about how this impacted her second pregnancy with her daughter, Mara said she and Bell were “definitely excited for sure, but there was this fear element as well”.
“We were both quietly excited and clearly very scared,” she said.
Bell also has an eight-year-old son named Jack from his previous marriage to Evan Rachel Wood.
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