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The u/s measures GA (gestational age) which is the age calculated from your LMP. It measures the size of the baby from crown to rump and compares to average lengths that baby is currently and comes up with a date. It is highly accurate within a few days. So in other words, if the u/s says 11 weeks, that is 11 weeks after LMP, but the baby was technically concieved 9 weeks ago. Your Dr will most likely go with LMP dating as a normal pregnancy lasts around 40 weeks (including the two before conception). It is totally normal to deliver any time between 38-42 weeks because of that...
Hope that helps!
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