I found an interesting theory about Juliet on PopCandy's Lost message board. Makes sense to me.
3. okgirl_ok wrote:
Wait, I've been thinking about this. let's not sell Juliet short on her deception abilities. She totally lied to Sun about the father. Why wouldn't she? She left a message to Ben that said the same news she'd given Sun, but if that was true, she's not getting off that island, baby. She'd want to tell Sun the same thing so her story would be able to corroborate. BUT, if she found out that Sun got pregnant OFF the island and LIED about it, then her experience about Claire would tell her- since the baby survived - she can settle her deal with Ben and get her butt gone. She's just making it look like she's fixing the problem and setting herself up to "Save" Sun from the island's conception-death problem.
Forger those others and that they still won't be able to get pregnant, she's every woman for herself. Plus the double bonus of getting to make Sun very happy about Jun being the father.
Why would Juliet say anything other than that Sun got pregnant on the island? She needs to prove that hero factor to everyone - otherwise, she's stuck there, baby.