No. An unborn sim doesn't have any genetics, in fact it's not even a "sim" until it's born, it's just a pregnancy token in the mother sim.
I know that, the only reason I thought this would be possible is because I *thought* I saw it covered somewhere before. Gah! That was a few years ago. Who knows what I originally saw. >_<
The idea of what I'm wanting to do doesn't have to involve "unborn DNA". Just a means of getting the game to use the DNA from a premade sim instead of generating it at random. I wouldn't know if that's possible though.
Or take one of the sims you like, age it down to newborn and use a hack for cloning over to clone it onto the newborn and then have the fairies take the doppleganger away somewhere - that'd be a lot more interesting.
Can you age sims in to newborns? I aged a sim from newborn to adult once to see what it would look like, but wasn't able to age him back, not past a toddler anyways. I was using insim.
Or, even more fun, if the baby turns out ugly and with a bad personality have the parents insist it must actually be a fae doppleganger and set out to prove it (throwing the infant in the fire, traditionally) and quest to get their 'real' baby back.
Oh god, I think you just gave me the inspiration I need for my next story