This made the rounds on my wife's babycenter forum. It's got to be a fake. The sheer amount and thickness of tissue between the baby and outside would prevent such detail from showing through. You probably couldn't even see that kind of detail if it were pushed against the amniotic sac.
Honestly, I'm pretty certain it can happen...mostly when the woman is having multiple births and the skin is stretched alot and taught...maybe I'm wrong though.
Well, I know when I was pregnant there was no way you could see that much detail, it was hard to tell if it was an elbow or a heel or a shoulder or what pressing out. Plus, I'm with orie, the footprint looks too big to me.