Have you had a scan before this one? That would be more accurate to compare the difference in dates.

By memory at 19weeks if you get an EDD from that the area for effort is said to be + or - 3weeks. So although you may indeed be 19weeks (or 17weeks since conception) the baby can be growing at a different rate so be smaller or larger then the average 19week baby. It makes sense when you think we are not all going to have an average 3.3-3.4kg baby. The bigger babies ted to measure a week plus ahead and the smaller ones less.

With my 2nd my 18week scan showed I was 21+ weeks. The EDD was 30th April although my 8week scan had given me an EDD of the 15th of May. She arrived at 36weeks weighing almost 8lb but clearly early (due to vernix, poor muscle tone and too tired to feed).