Assuming a fake conception date of 21 September (just before the crash), the fake due date would be 27 June 2005 (40 weeks). Lostpedia figures the actual conception date, or at least the one claimed by Juliet, to be around 24 October, which would put the actual due date around 31 July 2005. I'd say that as long as she gave birth sometime in July, no one would raise an eyebrow; obviously doctors would be able to tell the difference if they looked hard enough, but that's probably a risk the O6 is okay with taking. They could also claim stress or whatever as a factor too (they couldn't claim the island did it, because they're hiding the island and its properties).

Assuming Jack wasn't too drunk to do math, Christian's funeral did occur in July 2005 (ten months after leaving Sydney). In Sun's episode Hurley acted like relations with the other O6 were strained, and if this is true then either a) the funeral occurs before the baby is born or b) the O6, or at least Hurley, is sucking it up for Jack's benefit.