Ashlee, 30, gave birth to a daughter, her second child and her first with Evan, 26, on Thursday. The baby joins big brother Bronx, 6, whose father is the singer and actress' ex-husband and Fall Out Boy rocker Pete Wentz, 36.
"We are overjoyed about the birth of our precious baby girl Jagger Snow Ross," read a message posted on Ashlee and Evan's Instagram pages.
"The couple had their minds set on this name for a long time," a source told E! News exclusively. "They wanted something fun and different."
Bronx had told E! News earlier this year that he hoped to have a little sister.