I know your question is fantasy related and all,
but if you did want to have the pregnant woman taking on superpowers, it would be very possible to explain it by some Kryptonian hormones or other biological phenomenon (but I think the hormones would be best, as these easily cross between mother and child.... in fact that is the basis of pregnancy tests, as they measure hormones specific to the human embryo)
This takes away the magical aspect (that you got from charmed, or whatever)
but is stilll an a fantasy world, so would work without magic...
