The `Die Another Day` actress and her partner Robie Uniacke - who have been dating since December 2009 - recently welcomed son Solo into the world, according to the Mail On Sunday.
The 33-year-old actress kept her pregnancy under wraps until she was seven months along and could no longer hide her bump.
Although Rosamund and Robie - who already has four children - are not engaged, they have discussed moving in together.
A source previously said: "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."