Understanding Ultrasound Accuracy in Pregnancy - Week Calculation

Question
QUESTION: i got an ultrasound on jan 28,2011 it stated that i was 13wk and 5days i didnt tel them when my lmp was because i didnt memeber what day.... now i was wondering if that was true abut ultrasound adding an extra two weeks too pregnancy so that if i was 13wwks and 5days would count back 11wks and 5days is that true

ANSWER: I am not sure about your question. If you are asking if an USG adds two weeks to the age of pregnancy, then it is not so. The ultrasound like any other imaging modality will have some variability and error. The variability is less in the early weeks of pregnancy and increases towards the end. Therefore it is not uncommon to see two different ages of the fetus on repeat USG. For more information on ante natal ultrasounds visit http://www.pregnancyandcildcare.info/pregnancy_articles/anc.html  

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QUESTION: well i was just wondering because on y ultrasound it ha my lmp and the g.a of the baby as 11wks nd 5days an so when i counted that back it put me on the day of my lmp so thats what i meant by the extra two weeks including my period.... and the other ultrasound i got put me at 13wks and 5days which would have mad my lmp october 24th and she told me conceived at th beg of nov but i didnt knw if that method was the true

Answer
The sonologist is right. if your lmp was on 24 oct then you would have conceived some time in early November. What is you LMP. From there count the weeks and you should know the age of the pregnancy. Compare it with the USG reports. Also remember that it is entirely possible for two USG scan at different time is a pregnancy to show a variation of 2 weeks in the age of the pregnancy.