The singer was joined by her friends and family - including Jessica Alba and sister Ashlee Simpson - on Sunday afternoon in Los Angeles, People reports.
"Jessica looked pretty and very happy," a witness said.
"She's so excited to be a mum," a friend added. "She can't wait to meet her little girl."
The 31-year-old recently confessed that being pregnant has made her sex drive "unstoppable".
Earlier this month, Jessica denied rumours that she is expecting twins. The singer also revealed that her wedding plans are postponed until after her baby's birth.