The term "pregnant rollerskate" was CB and/or trucker slang for a Volkswagen Beetle, referring to both the shape (i.e., like a pregnant woman's belly) and the size (rather small) of the car.
The classic Beetle (aka the VW Bug) certainly has the right shape. The curve of the passenger compartment would be the belly, the headlights would be where the legs connect to the hips, and I'll let your mind go from there.
"Break One-Nine for the rolling parking lot with the pregnant roller skates, ya got yer ears on?"