It takes a very powerful Sith sorcerer to do such a spell. The Sith are rare, and not all Sith use spells, so, darkchrono, it'd be feasible and the reason there arent a lot of them... not to mention Sith are loathe to make it possible for anyone to be stronger than them, so again, they wouldnt do it very often.

While I do agree that Anakin is birthed by the Force itself, I'm merely stating the possiblity, as Sith sorcerers have that spell available to them.