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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.Ultrasound Dating Ultrasound can be used to date pregnancies, especially when the LMP is not known (for example, pregnancy after a delivery but before a menses occurred or irregular menses without predictable ovulation).Ultrasound dating is based on a series of measurements of the gestational sac in very early pregnancies and of the embryo or fetus itself as pregnancy progresses.One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly (excluding cases of assisted reproduction), but we can know the first day of LMP, based on what a patient reports to us.Also, when providers all use the same LMP date, we are using one system that provides a standard convention.
For example, if your LMP was November 1, 2017: In this example, the due date would be August 8, 2018.
However, unless there is a great difference between what is seen on an ultrasound and what was reported as LMP, doctors still use LMP to establish a due date.
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.
The expected duration of a pregnancy is 40 weeks from the first day of LMP (four weeks more than the traditional nine months by which many count).
Medical professionals consider delivery premature when it occurs before 36 weeks, and extremely premature when it occurs before 28 weeks’ LMP.