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2 Weeks diff. in gestation is possible, if you conceived 2 Weeks later than as expected by LMP. You have not mentioned about fetal cardiac activity, seen or not seen at the time of Ultrasound
Study. Normally even with Endovaginal ultrasound, fetal cardiac activity cannot be made out clearly at 5 wks 3 days. I think they must have suggested restudy after 1 week or more. Only when we see the fetal cardiac activity on Ultrasound can we be sure of viability of the pregnancy