Stomach measuring small (4 weeks behind) at 39 weeks

Anyone this far along start measuring 4 weeks behind? (Your belly) I'm about 39 weeks and measuring 35 weeks. I haven't had an official ultrasound since 18ish weeks for my anatomy scan. So we have no measurements of baby at all what so ever. Now I'm pretty stressed something is wrong. With my son around this time I started measuring a week behind due to dropping but not a whole 4 weeks. I'm being sent for a stat ultrasound but it's going to be some stressing out until then.