which due date do you follow?

my 2 previous pregnancies I always followed my LMP due date but with my 3rd and current one I have new doctor and she told me to follow the dating ultrasound which is supposed to be most accurate since I am not sure when I conceived but 100% sure of my LMP.. However i ended up in emergency this past weekend and he asked me when my last menstrual cycle was and i told him im 16 weeks and he said i dont want to know that, i want to know when your last period was, so i told him. He said your 17 weeks and not 16 like you think. He said dating ultrasounds are for those who dont know when their last period was.. Every baby can measure days, even weeks different on an ultrasound.

so now im confused which due date to follow.. my lmp and dating u/s are a whole week different..

my sister had about 4 different due dates.. every time she went for an ultrasound it changed.. cuz she was one of those women who had no clue of her last period or when she conceived .. her first was october 23rd, then october 19th then october 7th now its october 11th.. i just find that crazy! ..