The stunning actress, 29, was true to her name in a scarlet red cocktail dress with matching bright red lipstick and nails for the London premiere of Captain America: The Winter soldier.
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The red number from Vivienne Westwood's Red Label collection accentuated the star's voluptuous curves and revealed the beginnings of a tiny baby bump.
A stunning diamond necklace and a pair of leopard-print high heels finished off the fiery look.
Scarlett is expecting her first baby with French fiancé Romain Dauriac just six months after the couple got engaged. The pair first went public with their romance in November 2012.
Scarlett and co-star Chris Evans at the Thursday evening premiere
"I'm very happy," Scarlett, 29, told People in January of Romain, 31, a journalist who manages a creative agency. "He's my buddy."
Scarlett's producer Kevin Feige recently revealed her pregnany during an interview with Collider.com.
"Well my first reaction was I was very happy for her and very excited for her," he said of Scarlett's pregnancy news.
The actress and her French fiancé are expecting their first child together
"My second reaction was we've got to move some pieces on the chess board around. But we didn't change the script at all, which is sort of the most important thing for us in terms of the storyline," he said of The Avengers: Age of Ultron script.
"I think the goal is for us to sit here talking about the fact that you would never know that Black Widow was pregnant during the filming of the movie when you see the final version of Avengers 2."