Teefey, 37, welcomed the baby with Selena's stepdad, Brian Teefey. While Gomez, 20, hasn't announced the news just yet, she was seen leaving Northridge Hospital in Los Angeles, wearing sweats and UGGs on Thursday, June 13.
The "Come & Get It" singer's best friend, Demi Lovato, sent her congratulations to the family in a tweet.
@selenagomez IS A BIG SISTER!!!!!! Everyone congratulate her and her family!!!!!! #CONGRATSSELENA!!!!" the "Heart Attack" singer wrote on June 13. She later added, "#CONGRATSSELENAMANDYANDBRIAN !!!!!!"
The baby's arrival is happy news for the family, as Mandy suffered a miscarriage in December 2011, sadly losing a baby who would have been named Scarlett. In a Christmas 2011 post to her fans, Gomez referred to Scarlett as the family's "guardian angel."
On Mother's Day 2013, the former Wizards of Waverly Place star announced that her mom was expecting again. "Momma and Brian are finally letting me share the news...My mommy's carrying my baby brother or sister :) I'm the happiest girl in the world!" she wrote.
Mandy and Brian, who serves as Gomez's manager, tied the knot in 2006. The Texas-native Mandy divorced Gomez's dad, Ricardo Gomez, when the actress was 5 years old.
This article originally appeared on Usmagazine.com: Selena Gomez Becomes a Sister, Mom Mandy Teefey Gives Birth to Baby Girl