The couple - who already has a two-year-old daughter, Sunday Rose, together - welcomed Faith Margaret Kidman Urban into the world on December 28 last year at The Women's Hospital at Centennial in Nashville, USA. The tot was born through a gestational carrier and is the biological daughter of both Nicole and Keith.
They said in a statement: 'Our family is truly blessed, and just so thankful, to have been given the gift of baby Faith Margaret. No words can adequately convey the incredible gratitude that we feel for everyone who was so supportive throughout this process, in particular our gestational carrier.'
Kidman is also the mother, from her previous marriage to fellow actor Tom Cruise, to adopted children Isabella Jane, 18, and Connor Anthony, 16.
Golden Globe
The 'Rabbit Hole' actress and the country singer had kept the birth a secret and didn't mention their new arrival when they appeared on the red carpet at Sunday night's (16.01.11) Golden Globe Awards.
Kidman and her husband, both 43, seemed to have waited until after Sunday's Golden Globes award ceremony was over before making the announcement.
The couple attended the awards gala in Beverly Hills, where Kidman was nominated for her leading role in "Rabbit Hole." However, it was Natalie Portman who took home the win for best actress after her "Black Swan" performance.
Surrogacy
In a gestational surrogacy, the surrogate mother is carrying a baby that is genetically unrelated to her. She becomes pregnant through in-vitro fertilzation.
Kidman and Urban are the latest celebritiy couple to turn to surrogacy. Sex and the City's Sarah Jessica Parker and husband Mathrew Broderick added to their brood with the birth of twins Marion and Tabitha in June 2009 via a surrogate.
Elton John and partner David Furnish became parents to son Zachary in December through surrogacy, as did How I Met Your Mother star Neil Patrick Harris and partner David Burtka who welcomed twins Gideon and Harper in October last year.
Some estimates put the cost of surrogacy at between $60,000 and $90,000.
About 750 babies are born via surrogate in the US each year according to the New York Times and it is legal in the state of Tennessee where Kidman and Urban's baby was born.
The couple, both 43 and married in 2006, were overjoyed when Nicole gave birth to daughter Sunday Rose in July 2008. The name 'Sunday' piqued the curiosity of the media at that time. Some like baby expert Pam Satran, co-author of 'Cool Names' and 'The Baby Name Bible' attempted to explain .
"Sunday is one of the next generation of word names, explained Satran.
"A really surprising and unique choice - the name Sunday has hardly been heard since the 1940s, when there was a soap opera called Our Gal Sunday.
"Unlike months of the year (April, May, June) which are quite common, day names are rare, with the exception of Tuesday Weld and Wednesday on The Addams Family.
"Unusual as it is, Sunday does project a nice sunny, serene image, it being the day of rest.
Sunday Rose's middle name, Rose, is a tribute to Urban's late grandmother Rose.
Sources: xinmsn, news.com.au, dailytelegraph
Also read: Nicole Kidman reunites with estranged daughter Isabella Cruise