The "Come and Get It" singer, 20, shared the happy news on Twitter. "Hope all the amazing moms had a great day! I have the best momma in the world :)) I love you so much!" Gomez captioned a photo of herself cradling mother Mandy Teefey's baby bump.
It will be the first child together for Teefey and her husband Brian, whom she wed in 2006. The couple had previously been expecting, but sadly lost the baby.
The day prior, Gomez greeted 70 contest winners at LDV Hospitality's Rec Room in Miami Beach, where she discussed her plans for the holiday.
"Her mom wasn't able to come to Miami for this trip, but Selena was flying home and knew it would be an extra special Mother's Day for her mom," a source tells PEOPLE. "She said she got her mom a really great gift but it was a surprise and she couldn't wait to give it to her."
Teefey and Gomez's father Ricardo divorced when the Spring Breakers star was 5.
"I was frustrated that my parents weren't together, and never saw the light at the end of the tunnel where my mom was working hard to provide a better life for me," Gomez told Glamour. "It's because of [my mom] that I can do things on my own."
Reporting by KEVIN O'DONNELL