The 42-year-old singer and her husband Nick Cannon welcomed their first children into the world at 12.07pm 30.04.11 - their third wedding anniversary - with their daughter arriving first, weighing 5lb 3oz, and her 5lb 6oz brother arriving shortly afterwards.
Nick later tweeted his excitement about the new arrivals, whose names have not yet been released.
He posted: "My wife just gave me the most incredible anniversary gift ever in life! I won`t ever be able to top this!"
Mariah`s spokesperson Cindy Berger - who revealed the couple listened to the singer`s hit `We Belong Together` as the babies arrived - said the star is doing "really well" and was completely calm throughout her labour - unlike Nick, who struggled to find his way to the maternity unit at the undisclosed Los Angeles hospital where his wife gave birth.
Cindy said: "Nick was very nervous and Mariah was completely calm. Mariah thought it was another false labour and Nick was driving her to the hospital and she very calm and the music was blasting.
"When they walked into the hospital, Nick was going the wrong way and they nurse redirected them to go the maternity ward, and I said to her, `That`s right out of `I Love Lucy`,` and she laughed."