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A member asked:
At an ultrasound, what determines if you are 12 weeks or 12 weeks + 4 days? are measurements this precise? and the accuracy correlates with your lmp too?
A doctor has provided 1 answer
Crown Rump Length: At twelve week on ultrasound, the sonographer measures the fetus' crown rump length (crl). This measurement is from the base of skull to tailbone. Ultrasounds at twelve weeks are accurate within 7 days. If you were 12 weeks by LMP and 12 4/7 weeks by sono, you due date would stay consistent with your lmp.
Answered 2/13/2015
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