9wk 2 days pregnant, baby measured at 8 weeks

Dc

I had my first US today and the doctor told me that the baby is measuring at 8 wk and not 9 wk 2 days (based on lmp). The heart beat was 162 bpm. Should the size be a cause of concern? My doctor said there is nothing to worry but i still want to get some assurance from moms who have been in a situation like this and everything has been fine for them. Thanks.