Hindi exactly 40 weeks lang ang calculation, sis, usually EDD nila ay based sa last menstrual period mo, so around 280 days or 40 weeks from that date, but every pregnancy is different talaga. Yes, it's very possible na mabigyan ka ng December EDD pero mag-deliver ka ng November, marami sa amin na nangyari yan, some deliver earlier, some later, so ang EDD is really just an estimate lang, hindi guaranteed na exact date!