[quote=@ClocktowerEchos]
So it would just go *pop* and vanish? I'm not a biology expert or something like that but I'm fairly certain that it wouldn't work like that.
[/quote]

I don't think biology expertise would help. But it's kind of got to depend on at what point the baby is classified as part of its mother, and when it's considered an individual I guess. If you die under the influence of Polyjuice potion you stay dead, so major changes do stay. Maybe it'd burst out like a xenomorph? Expect from the crotch. THe real question here is if a pregnant mother takes polyjuice potion, does it pass onto her baby and transform it too?