[John gives Sherlock a frowning, 'now you're just being a dick' look.]

Of course I didn't think that. Even if I did, a dose of the potion only lasts a few hours at most. That's not even enough time for an actual woman to get pregnant; definitely not enough time for an egg to implant, maybe not even enough to conceive.

I wasn't so sure about the period thing because you physically, and from most accounts I've heard very completely, become the person you've based the potion on. And if that person was on her monthly when you nabbed her hair for the potion... there's worse leaps of logic to make. And I already said it was far-fetched and that I'm no potions expert, so there's no need to be so snippy.