Hi, Thanks for writing in. There is some discrepancy in pregnancy age between the dates of your last period and the ultrasound age. This can happen but should always be confirmed by repeat ultrasound done 15 days later to get the real age of the pregnancy. It will also show the presence of heart beat which can confirm viability of the fetus. As per present ultrasound, the size of sac is 8 mm and this is the usual size at 5 weeks. There is no need to get worried but please get a follow up ultrasound in 15 days to make things clear.