Calculator: Preterm Z-scores, 22-50 weeks

This page is running a macro package for R hosted on a Shiny server. For weight, length, and head circumference for gestational ages 22-49 weeks, it calculates Z-scores and percentiles based on reference data published by Fenton, 2013

The 2013 manuscript describes two different growth references based on either completed weeks , where a gestational age of 24.6 weeks is rounded down to 24 completed weeks, or actual weeks = days/7. Due to the way the time axes were translated, 24 completed weeks is numerically equivalent to 24.5 actual weeks and not 24.0 weeks as you might expect. For this reason, valid LMS parameters are available for weight (22-49 weeks completed, 22.5-50 weeks actual), length (23-49 weeks completed, 23.5-50 weeks actual), and head circumference (23-49 weeks completed, 23.5-50 weeks actual). While completed weeks were favored historically, actual weeks follow the charted percentiles more closely; you can make your selection in the sidebar.

The sample.csv file shows the expected variable names and formats. Column order is immaterial, and additional columns are permitted. The identifier variable id must be unique for each observation. The variable sex may be coded as M/F or m/f or 1/2 (1 = male); length and head circumference are in cm, weight is in grams, and weeks is age in weeks (non-integer values will be rounded down for completed weeks). To ensure consistency with WHO recommendations for skew distributions, weight Z scores outside of the range [-3, 3] are calculated in units of SD23, the distance between Z=2 and Z=3 (or in the lower tail of the distribution, Z = -2 and Z = -3). This adjustment does not apply to weight percentiles 0.1-99.9%.


A spreadsheet with comma separated variables (.csv) may be created using the 'Save As' .csv option in Excel and uploaded using the sidebar on the left. The preterm growth chart plotter can also be used to create a suitable .csv file as you enter data for charting. For assistance, you can also consult our step-by-step guide: Creating .csv spreadsheets . Once results are displayed, download them by clicking the <Download> button, which will typically save them to your Download folder with 'out_' prepended to the name of the original dataset


Download Results