Well so far we havent been told why pregnant women die on the island, but its clear that this hasn't always been the case as ethan was born there i can only speculate that perhaps, the pregnant women on the island only started having complications after the "incident", whatever that may be...

Simply put Whatever happens Happened!!!

Pregnant women only start dying AFTER Ben killed all the Dharma Initiative.. maybe the island was angry, and that was the punishment. Or maybe she got pregnant before coming to the island.. because didn't it say in the season where Sun got pregnant that if she got pregnant before coming to the island she would be safe, but if she got pregnant on the island she would be in danger.

30 years ago Juliet was there, she (and the others that are back in dharma time) are only just now experiencing it. Even though this is her present it's all already happened.