Peaches Geldof -- OOPS I Dumped My Baby

Someone get Peaches Geldof's baby a helmet ... because his 23-year-old mom just dumped his ass out of his stroller while blabbing on her cell phone in London ... and she didn't even hang up.

Peaches seemed to have the whole stroller-pushing thing down pat -- until she hit a pothole, and the stroller flipped upside down ... with her 5-month-old baby still inside.

The kid tumbled out onto the sidewalk and Peaches jumped into action -- all without hanging up her phone. As far as we know the baby's fine ... for now.

Peaches gave birth to Astala Dylan Willow Geldof-Cohen in April. The boy's father is British punk singer Thomas Cohen (from the band S.C.U.M.).