Although there's been no official announcement, a source in America has told US Weekly that the 30-year-old actress and her boyfriend, Tom Sturridge, 26, are excited about being parents for the first time.
"Friends are really happy with her relationship with Tom. He's younger, yes, but he's mature and he's a good guy.
"We're not surprised by the news of the pregnancy, they're really good together. They just spent the holidays in Paris together after they announced the news in London."
The source also reckons the possible parents-to-be have no plans to marry at the moment.
What do you reckon?
Could you see Sienna with a (very stylish) bump?