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Chapter 8 Measurements and Calculations
Tips!
Standard Deviation & Percentile
Among the OB information, the Growth table and the typical fetal distribution, for the same number
of weeks, are used to determine the following information:
X
The normal distribution curve.
X
The measurements for an actual fetus or a position in EFW distribution.
X
Whether a distribution point is within the normal range.
The number of weeks referenced for the Growth table can be set to LMP, Estab. Due Date, or Average
US GA, under Pctl. Criteria. The typical setting is LMP.
When the LMP is not known or uncertain, or when the difference between the LMP and the Average
US GA is substantial, care must be taken, as selecting different Pctl. Criteria can result in a significant
difference.
The distribution of the number of weeks in the Growth table for the selected reference is a normal
distribution. It is laterally symmetrical around 50% (the average), and it shows the distance from the
average as a deviation. The deviation can be represented by Standard Deviation (SD) or Percentile.
[Figure 8.12 The distribution of the Growth Table for the selected week (m: mean, σ: standard deviation)]
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