An estimated fetal weight on its own doesn't say much — 1,800 grams is unremarkable at 36 weeks but a significant growth concern at 30 weeks. This calculator converts an EFW and gestational age into a percentile, so the number is judged against fetuses at the same point in pregnancy, and flags whether it falls in the small- or large-for-gestational-age range.
How the percentile is calculated
The calculator interpolates directly against a published reference table with 22 percentile columns (1st through 97th) for each completed gestational week from 24 through 41. The table is built on Hadlock's mean fetal-weight-by-gestational-age regression, with the customised-growth-chart percentile derivation of Gardosi et al. and the percentile/SD methodology detailed by Mikolajczyk et al. — a cross-referenced derivation used in preference to the original 1991 Hadlock table, since later research identified internal inconsistencies between that manuscript's published equation and its published table.
Interpolating directly against real percentile columns, rather than fitting a normal distribution and computing a z-score, keeps the result faithful to the actual shape of the published data, which is not perfectly symmetric. Values that fall between two tabulated weeks are linearly interpolated; values beyond the 1st or 97th column are linearly extrapolated from the nearest two columns.
SGA, LGA, and what the flag does and doesn't mean
Small-for-gestational-age (SGA, below the 10th percentile) and large-for-gestational-age (LGA, above the 90th percentile) are the standard clinical screening cutoffs. Both are exactly that — screening flags, not diagnoses. SGA raises the question of fetal growth restriction (FGR/IUGR), which is confirmed only by weighing the growth trend across serial scans, amniotic fluid volume, umbilical artery Doppler studies, and maternal risk factors together. LGA is associated with conditions like gestational diabetes and influences delivery planning conversations, but macrosomia is a clinical diagnosis, not a percentile cutoff alone.
Limits and edge cases
The reference table only covers 24w0d through 41w6d — the calculator will not guess a percentile outside that window. Different growth standards (this Hadlock-based reference vs. INTERGROWTH-21st vs. a locally customised chart) can produce slightly different percentiles for the same EFW and GA, so the exact number should be read as an estimate, not an exact value. And because EFW itself is an ultrasound-derived estimate with its own margin of error, a single percentile reading is one data point in a larger clinical picture, not a standalone verdict — that picture is the treating obstetric provider's to assemble.