A concise method of calculating fetal weight percentile.

Objective: Assessment of fetal growth used in clinical decision making for managing pregnancies is based on percentiles relative to normal values for gestational age. Most clinicians utilize visual interpolation of look-up tables or graphs, which is neither accurate nor convenient. Calculation of growth percentile by numerical integration formulas requires digital computers with advanced computational capabilities and/or the storage of large data tables.

Study Design: Using established ultrasound measurement formulas and published fetal weight/gestational age tables, mean weights and standard deviations for gestational ages from 10 to 44 weeks were determined. From this Z-scores were calculated for the gestational ages. Z-scores plotted against 1st through 50th percentiles resulted in a curve. A polynomial equation was derived to fit the curve resulting in an expression for converting the Z-score to percentile: Percentile = 0.0994 * Z score2 - 0.44 * Z score + 0.5026.

Results: Comparing calculated values from this equation against standard percentiles from a Z-score to percentile table, all calculated values varied from the table value by < 1 percentile for Z-scores ranging from 0 to 2.33. Values > 2.33 are assigned to the 99th percentile. The equation performed to the goal of calculating percentile to an accuracy of < or = 1 percentile.

Conclusion: A method whereby the estimated fetal weight can be used to calculate a percentile using two simple equations and only standard math functions allows implementation on a standard personal digital assistant device commonly available for clinical use. This implementation gives the needed accuracy, ease of interpretation, and portability, making it a useful tool in clinical practice.