Ultra sounds in determing fetal age

I'm very confused. I am 13 weeks 3 days pregnant today. I have a habit of writing down everything: periods, expected ovulation dates, even when my husband and I have sex. My last period was on Nov 17 it was a normal period. My husband and I planned to conceive on Dec 1.

Today I went for my first ultrasound and I was told that the baby's crl was 15wks 1day.
My doc told me that I must have been wrong about my last period.
I was told that ultrasounds this early are very accurate yet I can't understand why I'm being told that I'm nearly 2 weeks further along than I should be.
I took a pregnancy test on Nov 13th and it was neg I then got my period 4 days later.

Basically, my question is it possible that my baby is just larger based on family genetics or does this mean that I had (what seemed to be a very normal, average) period while I was already pregnant?
Should I go with what the ultrasound says or go with my own dates?

[alaysha]

I know that ultrasounds can be off by a week and based on your LMP you would be 14 weeks tomorrow. Its possible that the baby is measuring a little big right now and that is why they are estimating it bigger. Babies all grow at the same rate until 13 weeks and then they start growing at their own rate. Using ultrasounds for dating is VERY accurate but only in the beginning weeks of pregnancy.

[nikki23512]

ultra sound in the first timester is very accurate indeed people think they only ovulate on day 14 which is not true with all this new technology i would defo go with ultra sound dating which is more accurate than lmp ultrasound in the second timester is off by 2 weeks and 3rd timester 3 weeks