"We've spoken a lot and I know she will be amazing," Sienna told the New York Post. "She doesn't need any advice. She will be perfect."
Sienna says Keira will be a "perfect" mother
The silver screen stars have remained good friends since they starred alongside Cillian Murphy in 2008 film The Edge of Love; which told the story of poet Dylan Thomas's famous loves Vera Phillips, played by Keira, and Caitlin Macnamara, played by Sienna.
While she was full of praise for her friend's maternal instinct, New York-born Sienna will no doubt be on hand to support her friend if need be.
Sienna, who was one of only 11 guests that Keira invited to her wedding in 2013, is mum to daughter Marlowe with her actor fiancé Tom Sturridge.
The duo have been friends since starring in The Edge of Love
Keira, meanwhile, is married to Klaxons musician James Righton. The pair recently announced that they are expecting their first baby in 2015.
The Bend it Like Beckham actress confirmed to Elle UK that rumous she is expecting are true, days after suspicions were raised when she was seen avoiding alcohol at the Moet British Independent Film Awards.