After giving birth to son Bronx an entire 18 days ago, Ashlee Simpson did what every new mom does ... she went clubbing in West Hollywood.

Ash checked out her husband Pete Wentz's show at the Troubadour and then hightailed it next door to Foxtail for a little see and be scene action.

Hopefully, the 24-year-old will be as good a mom as she is a singer.