Understanding Gestational Age: Ultrasound Accuracy & Dating

Question
Hello,

According to my doctor I am exactly 12 weeks pregnant and likely conceived around the 2nd of September. My last period was 8/20/10 and the one before that was 7/20/10. I went to get my first trimester screening on 11/12/10 and the gestational age listed on the image was 12wks 0days. My question is:1) how accurate is the conception date provided by my doctor? 2)is the gestational date calculated based on the ultrasound or is it put in by the ultrasound tech based on my reported LMP? I am very curious to know when I conceived and how far along I am because my lifestyle prior to the pregnancy was not exactly ideal (i drank and smoked pretty regularly) and I did not find out about it right away. My periods have always been pretty regular, coming every 29-31 days, but everyone's memory fails them now and then, and these days mine is on the fritz more than ever. I'm worried that the date I gave for my period in August may have been off by a few days or that it wasn't a "true period" and just early bleeding. Any help you can give me would be greatly appreciated. Thank you.

Answer
Hi Alex,

It looks as though the dates are accurate with your doctor.  The gestational age of the baby would not be changed if the baby measured within 5 days more or less than 12 weeks 0 days.  So if on the ultrasound your baby measured 12 weeks 2 days, it is still in range of that LMP date and would remain at 12 weeks 0 days.  So, having said that your true LMP could very well be off by a little.  When I was pregnant I also had to give my best guess and my first ultrasound I got was 4 days off from my LMP.   My due date still stayed the same as my LMP.   Keep in mind,  ovulation can vary woman to woman and month to month, and only 5% of full term births are on their given due date.  

Take care!
Sara