I made up the potion, paused the game, rounded up my 8 women in the bath and set them to parenting. Then while it was paused I dropped them all at the cauldron. There was one joint 'slurp' and a magic flash. then she turned around with the baby. So perhaps it just consolidated the animation?
Well, I don't think the potion gave her the child; I've never heard of that, and I know most of the potions. The fertility potion in particular does not seem to be a guarantee of getting a child, even if the villagers do go indoors.
Was there another male in or near the bath, that might already have been heading indoors? Perhaps your lady villager had just been propositioned successfully. When you drag away a woman that is heading indoors, and is about to come out with a child, she will stand there a moment before the child appears. I think that you grabbed a woman heading for motherhood with Mr. Right, and the baby only showed after the potion-drinking animation was completed.
Just my guess.