Inside Ashlee's 'White-Trash' Birthday Bash

By Marisa Laudadio

10/07/2008 at 02:45 PM EDT

Ashlee Simpson-Wentz 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