10/07/2008 AT 02:45 PM EDT
traded her stylish maternity wardrobe for trailer-park chic Friday night, at a costume party to celebrate her 24th birthday.
"It had a white-trash theme," says a guest who attended the bash at Simpson's parents' home in Encino. "Ashlee wore Daisy Dukes and platform flip-flops with a bathing suit and a fake tattoo around her belly."
The singer celebrated with friends and family including husband Pete Wentz
, dad Joe and sister Jessica
"Joe wore a cut-off muscle T-shirt and a mullet wig. It was funny," says a guest. "Jessica was wearing a crazy leopard-print dress that showed off a lot of cleavage."
The menu included corn dogs, macaroni and cheese, chicken fingers and French fries.
With her first child due next month, an exhausted Simpson-Wentz headed home early, missing party guests Christina Aguilera
, Nicole Richie
and Joel Madden, who dropped by after hitting Gwen Stefani
's birthday party earlier that night
"Ashlee loves themed parties and costumes," says a friend. Last year, guests sported prom dresses, vintage tuxedos and fingerless gloves as Simpson-Wentz celebrated her birthday with an '80s bash
For more on the birthday party, check out the new issue of PEOPLE, on stands Friday Reporting by JENNIFER GARCIA