They have been on the island for 69 days. Sun took her pregnancy test in Episode 2-16, "The Whole Truth", which took place on about day 58, and even that date is friendly (it's probably 60+). She would have already missed a menstrual cycle had she conceived prior to the plane crash. There's no telling how long before Sun and Jin leave after Jae's death, but I assume it's about a week. That's 65 days from possible conception to Sun's realization that she's pregnant. That leaves no doubt that there must be another menstrual cycle in there that would be late had Sun gotten pregnant prior to getting on the island.
EDIT: This assumes of course that Sun has a normal reproductive system. Some women do bleed in the early stages of a pregnancy, anyway, but it's pretty rare.








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