Sexual intercourse with a pregnant woman, could also be used to describe sex involving chubby chasers/chunky monkeys.
John: Where the fuck were you last night, you wanker?
Mike: Rubbing tummies with Shelia mate, she is her final trimester and it is fucking weird!
John: Wow, really? you seem pretty evenly matched then, eh!
Tammy:Did you see the girl with Mike last night? She weighed like 15 stone!
Cheryl: Yeah, Mike prolly loves rubbing tummies!