How do I know how far along I am?

The first day of my last period was Dec 18th...I expected my period on the 15th and missed it, took a hpt on the 16th and got my BFP...I went to the doctor Monday and they confirmed my pregnancy and said that I was 4 weeks. I know that they calculate from the first day of my last period, but that is not how far along I am physically because obviously I didn't get pregnant the first day of my last period. I estimate it was around the 1st when I conceived. I hope this doesn't sound like a stupid question. I just dont understand why they do it like this.