You have two theories there. The first one, that magical pregnancy is a power-drain, is reasonable without qualification. And a truly powerful witch would either still be able to work magic *or* she can find a way to get some magic from her partner - which means that a less-powerful witch with a Muggle husband, or one without a mate, would in fact be almost a Squib. (Not quite - I suspect she could light her wand and see things like Dementors.) Unless Tonks or Fleur gets pregnant (and let it be Fleur, please), we probably won't find out. Merope's father called her a Squib. She, of course, isn't, but she could have low levels of power, so it's all possible. Perhaps loss of power only comes to near-Squibs anyway.

On the other hand. Magic is genetic. This is from Rowling. If a baby lacks the magic gene then there is no way for the child to become magical no matter how powerful the mother is (however, it might be a clue. "Darling, I'm worried. I feel no drain whatsoever." "That's all right, dear. We'll love it anyway/foist it on Muggles and try again."

Actually, it works. Because if a magical baby needs to have an influx of magic, then Muggleborn babies had better be around some source of magic in utero, even if it's not Mom. Otherwise, the mothers would miscarry, and never know why. Either that, or it needs to be around some sort of magic to *trigger* the gene. Either one works. I'm less fond of a magical trigger because then it's DC comics and the metagene.

But also - any teachers who get pregnant? Would be at Hogwarts. I'd think there'd be enough magic in the environment to make up for any drain.

After all, while we know where the ability to work magic comes from, we don't know where the magic itself originates.