The 40-year-old Australian star had already spoken of "winding down" her film-making activities as much as possible next year.
The baby, whose expected arrival date has not yet been revealed, will be the actress's first natural child.
She adopted Isabella, 14, and twelve-year-old Conor during her first marriage to Tom Cruise.
Miss Kidman was stunned when he filed for divorce in 2001.
Shortly after they split, she revealed that she had suffered an ectopic pregnanacy and a miscarriage during the marriage.
Miss Kidman spent her Christmas holiday in Syndey, after finshing work on Baz Lurhmann's epic romantic adventure film "Australia" on December 21.
She is due to film The Reader with Ralph Fiennes and British director Stephen Daldry in Berlin next month, but has no more cinema work planned after that.
"I just want to spend as much time as possible with my husband," she told the Daily Mail.
Earlier this month the actress also dismissed the idea of returning to the stage next year.
"It wouldn't fit in with the logistics and dynamics of my family", she said - a remark given extra meaning by the news of her pregnancy.
Her most recently-released film is the extravagant fantasy The Golden Compass.
Although the film was attacked by some critics, the actress's performance as the glamorous villain Mrs Coulter won wide praise.
Miss Kidman married Urban in Sydney in June 2006. (Daily Mail)