The actress and her boyfriend Tom Sturridge are expecting a baby together, Us Weekly reports.
A source told the magazine: "[Friends] were 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."
They added that the couple "don't have plans right away to marry, if at all".
A representative for Sturridge said: "We don't comment on our client's personal lives at all."
Miller's representative could not be reached to confirm or deny the report.
The actress was first linked to the Boat That Rocked star, who is also a friend of Robert Pattinson's, last February, soon after she split from her on-off partner Jude Law.
In August, the couple were said to have moved into a £2.9 million ($4.5 million) home in north London.
Last November, Miller gave evidence to the Leveson Inquiry into press ethics, reporting that she had felt "complete anxiety and paranoia" as a result of the attentions of the media.