we should start by asking if you are doing the ultrasound by the same doctor, and by the same machine.. Because you can have variability only by this...
If yes, when the "fetus" starts loosing weight, it means there is a problem somewhere!!
It may be due to abnormal placental function explicited by high blood pressure, preeclampsia ( protein in urine + high blood pressure);
Also if you are smoking, or consuming alcohol... Or you are on a diet..It can your fetus loosing weight or we called IUGR ( intrauterine growth retardation)..
As your doctor is implying, whenever we have this and you are at term(I.e after 36 weeks of amenorrhea) the best treatment is DELIVERY
So no you should not wait..