one other reason that makes me think Juliette might be lying is that, like Sun noticed it quickly, Juliette asked on the way to the medical hatch, about the last time Sun & Jin had sex. Sun refused to talk about it and said the ultrasound would tell it anyways. Perhaps Juliette was fishing for more information to carry her plan? I know Sun's belly looks quite flat for pre-island conception but she's a petite Asian lady.

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"Bear...? Is that you?"-- Hurley S03E4