Dating a pregnancy
In the second trimester, the determination also defaults to the LMP unless the ultrasound estimate is more than two weeks different from the LMP.We follow the same process in the third trimester, with a three-week difference.
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Together, they cited information from 13 references.
If the ultrasound estimate differs by more than one week from the LMP, we would use the ultrasound estimate for the due date.
This method continues in the subsequent trimesters.