So, let's look on the bright side about not having many definite answers to some of the big mysteries: we all still get to discuss our OWN ideas!
Eh? Eh?! Perhaps I am too much of an optimist.
Anyway, given that:
-The Smoke Monster has been shown to be able to split himself into multiple entities, and -The Sickness that seemed to possess some of the characters in the show (Sayid, Claire, Montand and crew) has always been closely associated with the Smoke Monster, and -One's body temporarily losing possession of one's Consciousness has been shown possible in the Lost universe,
I'd like to put forth the theory that the Smoke Monster had found a way to somehow partially inhabit and possess people who's Consciousnesses were temporarily lost through a near-death experience. With Sayid, this would be the gunshot and near-drowning; with Claire, this would be the explosion that rocked her house at the barracks in Season Four. People with a strong willpower could fight against this influence, sometimes returning to their true nature (like Sayid and Claire).
How would this explain the pregnancy deaths? Maybe, sometime after the 70's, the Smoke Monster came up with the idea to try to possess people before they were even born, when their Consciousness was still weak as it formed in the womb. However, this never worked, as the mother's body recognized the other entity within it and attacked it with the immune system, killing child and mother alike.
What are your thoughts on my theory, and what might be your own? I am in agreement with many that the "conclusive answers" to some of the mysteries were either rather lame or even contradictory with other facts within the show, but I miss the days where people enjoyed discussing their ideas about the mysteries instead of ranting about how they were either left unresolved or were contradictory/stupid. So before the DVD comes out and Darlton answer mysteries with deleted scenes or commentary, let's enjoy the freedom of our own imaginations!