A source told The Mail on Sunday newspaper: "Rosamund doesn't look it but is seven months pregnant and she is glowing. She is looking forward to the baby in the spring."
Although Rosamund, 33, and 49-year-old Robie - who already has four children - are not engaged, they have discussed moving in together.
The source continued: "They are talking about getting a family home when the baby is born, although marriage isn't on the cards. Rosamund is so excited about the baby, and she's planning to take a break from work.
"She and Robie are very happy together. Robie gets a bad press but he is a great guy and adores her."
Rosamund - whose engagement to director Joe wright was called off a month before their wedding in 2008 - has always said she would like to become a mother and would like more than one child.
She said: "As an only child, you've never known anything else. But I definitely know that I would like my child to have brothers or sisters. But I haven't got a child yet, not yet obviously."
