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This method continues in the subsequent trimesters.In the second trimester, the determination also defaults to the LMP unless the ultrasound estimate is more than two weeks different from the LMP.

Trimesters, however, have a variability of a few weeks, so they should be thought of as general benchmarks.

The first trimester is generally considered to comprise the time up to 12 or 14 weeks from the LMP.

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For example, if in the first trimester the ultrasound estimate falls within one week of the LMP estimate, we still use LMP to determine due date and length of pregnancy.

If the ultrasound estimate differs by more than one week from the LMP, we would use the ultrasound estimate for the due date.

Some women will deliver on their due date, others before, and others after.

The body is not on a clock counting down to delivery; natural variation will introduce uncertainty.

Also, when providers all use the same LMP date, we are using one system that provides a standard convention.

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