They count 40 weeks starting from the first day
of your last period. As most women ovulate about 2 weeks after the first day
of last period, but not everyone, thats just a way for the Dr.s to count. You are really only actually "pregnant" for 38 weeks. So assuming the person in question always has a 28 day cycle, and always ovulates around day 14 (which is not always the case for everyone), then someone the Dr.