The singer welcomed her first child with husband Brandon Blackstock on Thursday and announced the happy news on Saturday.Taking to Twitter the Since U Been Gone hitmaker wrote: "Our baby girl River Rose Blackstock arrived on June 12th! Thank you everyone for all of your well wishes! Brandon and I are on cloud 9!!"Clarkson confirmed she was pregnant four weeks after she and Blackstock wed in Tennessee in October, and she revealed the baby was a girl in January, tweeting: "I knew it! Only a girl could cause this much drama with all this vomiting ha!"