According to Keith Urban, his superstar wife Nicole Kidman is "very, very pregnant", raising speculation that the birth of the couple's first child is imminent.

The country music star, performing with Kenny Chesney at a concert in the couple's US home of Nashville on Saturday, hinted to the crowd of nearly 50,000 that he expects to be a dad very, very soon.

"I'm going to dedicate a song to my very, very, very, very, VERY pregnant wife!" Urban said with a laugh, before singing his hit, Better Half, people.com reported.

The 41-year-old Kidman slipped backstage at Nashville's LP Field just after Urban began his hour-long set.

"Well, we're home, Nashville," a buoyant Urban yelled to the screaming crowd during the concert.

After his set, Urban and Kidman walked hand-in-hand, pausing briefly to wave to the crowd, to a waiting golf cart that whisked them away.

Meanwhile, OK magazine says Kidman's parents, Janelle and Antony, have arrived in Nashville ahead of the birth.

The star will be giving birth close to her Nashville home and has reportedly compiled her favourite music, mostly by flautist Sir James Galway, to listen to in hospital.

Kidman has two adopted children, Isabella, 14, and Connor, 12, from her first marriage to actor Tom Cruise.

AAP