Hun, having a dating u/s at 18-20 weeks is not that reliable - the earlier the scan the better for dating purposes - unless you have an idea from your cycle.

EG - my babies have been good sizes and have both measured ahead at the morph scan. This time when I was supposed to be 19w2d by dates, the scan averaged 20w1d - but I knew when I ovulated so I know I am not a whole week out etc.

So IMO don't hang your heart on the date they give you at the u/s for the due date. And - Good Luck