Understanding Conception & Pregnancy Timeline: Ultrasound & Due Date Discrepancies

Question
Help if u can. This is a little sticky situation. 1st baby, no compilations..first ultrasound put my due date 12/25/2010 by what I think was my last period. The size was .86 cm crl.  I can't tell if the size was accurate for being 6 1/2 weeks. Doc estimated my conceived date 3/30 or 31st. Can my conceived date be the same day I had sex? My other issue is my son came earlier on his own 12/16/2010 I was in labor 14hrs.  Could my conceived date be wrong? Was the size @ 6 1/2 weeks right or was I further along? Can you help figure out when I conceived?  Please and Thank you.

Answer
An usg done at 6 1/2 weeks can under or overestimate the age of the pregnancy by two weeks. This is because the foetus is not developed to permit an accurate dating. But in spite of this the date as derived by the Crown Rump Length is considered the gold standard. But even then your son was delivered two weeks ahead of the date,which is not abnormal. I mean a child is delivered any time between 38-40 weeks in 80% of pregnancies. So if you ask me, I will still put your conception date around 30 or 31 of march.