British actress Sienna Miller has joined the growing list of celebrities to give birth this year after she reportedly revealed that she is pregnant with her first child.

According to Us Weekly, the 30-year-old Alfie star and her boyfriend fellow actor Tom Sturridge announced the news to loved ones over the Christmas period.

The couple, who have been dating since early 2011, are said to excited about becoming parents despite the pregnancy being unplanned and unexpected.

A source close the star told the magazine: "(Their 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," the source added.