It will be the first child for both Cyrus, 22, and Song, 23, who began dating in May 2010.
Before the pair hooked up, they had been linked to fellow members of the Disney dynasty. Cyrus dated former "Sonny with a Chance" star Demi Lovato in 2009 and Song had been romanced by singer Joe Jonas in 2008.
Song is known for playing socialite London Tipton on the Disney sitcom "The Suite Life of Zack and Cody," the show's spin-off "The Suite Life on Deck" as well as "The Suite Life Movie."
She made her foray into more adult roles in the 2010 movie "The Social Network," playing the girlfriend of Facebook co-founder Eduardo Saverin in the Oscar winning film.
A frequent musical collaborator with his little sister and father, Billy Ray Cyrus, Trace Cyrus is currently working on an album with his own band Ashland HIGH.