"Happy 1st Day of Summer!" she captioned the snap.
Aguilera, 33, is expecting her first child with fiancé Matt Rutler, whom she met on the set of Burlesque. The "Beautiful" singer and Voice coach has a 6-year-old son, Max, with former husband Jordan Bratman.
Celebrity Families of 2014 [PHOTOS]
Jared Leto investing in Reddit stock