Why do pregnant women on the island always loose there baby in the 3rd trimester?

Thats a question that hasn't been answered in the show as of yet.
But one important thing to keep in mind is that it seems there wasn't always trouble with birthing babies on the island. Back in the 70's baby Ethan was born perfectly alright(in La Fleur i think). So it just raises the question what happens between the 70's and now on the island that caused all of this baby trouble.

I'm sure this'll be something they talk about in the 6th season. :)

This was never answered. I've seen people theorising that radiation from the incident could have caused it, or it could be a weird side effect of the island's healing power, but neither of these ideas work for me. I don't know if the writers actually intended one of them, and didn't realise that it doesn't make sense.