I was watching "One of Us," a Juliet flashback episode from Season 3. In this episode Juliet tells Jack and Kate that women can't give birth on the island because the mother's body turns on the pregnancy and treats it as a "foreign invader."
I believe the man in black is the “foreign invader,” hoping to spread the same sickness to these babies that apparently has taken over Sayid. Because of this, Jacob kills the fetus to avoid evil spreading on the island, and potentially into the world.
The reason Claire was able to give birth to Aaron and the reason Juliet successfully delivered Ethan in “LaFleur” was because Jacob looked at both of them as candidates. If you look at the cave wall in “The Substitute,” both Littleton and Goodspeed are written on the wall, proving they were once candidates.
Smccaffrey 20:18, April 22, 2010 (UTC)