Unexpected Pregnancy Discovery: Ultrasound Reveals Gestation & Date Discrepancy

Question
Hello, I am pregnant with my first baby and did not find out untill I was in the second trimester due to being under the impresssion by medical proffessionals that I could not conceive. I found out by ultrasound, the purpose of the ultrasound was to exclude polycystic ovaries and instead it turned out I was 12 weeks and 6 days pregnant. This makes me 33 weeks now, However this doesnt correspond with my LMP and I have one EDD written down "dates" and it says 26/6/2005 and one underneath that says "calculated by scan" which is the 22nd of july 2005. we are working off of the one which is calculated by the ultrasound but I have not been given an indication of the accuracy of a dating ultrasound first performed at 13 weeks. I have noticed a lot of braxton hicks that are starting to get more painful and leaking colostrum in the last few weeks and am wondering if maybe I could be further along than 33 weeks. Can you tell me if you know how likely it is that I could be 35 weeks or something around there? and what is the usual accuracy rate of any dating ultrasound? Thanks so much for your help.

Answer
If you are now 33 weeks based on the sonogram that was done at 12-13 weeks gestation, you are due on August 26 and conceived on December 1. A sonogram is quite accurate at 13-15 weeks gestation. An expected date of delivery based on "dates" is calculated from the first day of your last period. A date calculated by scan is based on the sonographic measurement. Remember that there is still an error of around 14 days in either direction using a sonographic measurement.At this point, a sonogram is not accurate in showing how far pregnant you are now.However, a songogram done at 13 weeks is very accurate.