Originally Posted by Inverarity
Polyjuice: It depends on how you decide the magic works, but I think Harry Potter magic rarely follows any sort of rigorous (or even non-rigorous) system of pseudo-scientific rules. It does what works thematically. I think Polyjuice Potion will make you look like the person whose hair you took, most likely at the time you took it. If you take the hair of a pregnant woman, then you're going to look pregnant, because the magic is going to magically make you look like her. That's what the spell to create the potion does: "Make me look like this person."