Ashton debuted her growing baby bump at the premier of her new movie, Mr Malcom’s List, in New York City on 29 June.
Arriving at the DGA Theater on Wednesday evening, Ashton opted for an off-the-shoulder cape gown by Sabina Bilenko Couture.
The full-length dress featured floral appliques across the arms and was adorned with diamantes.
Ashton wore her hair swept back in a half up half down style, and minimal makeup completed the look.
Hiddleston did not make an appearance at the event, but Ashton was joined by co-stars including Theo James, Freida Pinto and Sope Dirisu.
The news, which was confirmed by an anonymous source to Page Six, comes just two weeks after Hiddleston revealed the actors had become engaged in March.
In an interview with the Los Angeles Times on 14 June, Hiddleston said: “I’m very happy.”
Hiddleston and Ashton have been together since they played husband and wife in the 2019 West End play Betrayal.
They made their first red-carpet debut as a couple in September 2021 at the Tony Awards in New York City.
Rumours that the couple had become engaged first surfaced after Ashton was spotted wearing what appeared to be an engagement ring at the Baftas in March.
A photograph shared by presenter AJ Odudu to Instagram showed Ashton wearing a large diamond ring on her wedding finger.
Hiddleston, who famously enjoyed a brief fling with Taylor Swift in 2016, has previously said that he does not like to discuss his relationships in public.
“Everyone is entitled to a private life. I love what I do and I dedicate myself with absolute commitment to making great art and great entertainment, and in my mind I don’t conflict the two,” he told The Telegraph in 2017.
“My work is in the public sphere and I have a private life. And those two things are separate.”