NICHD Day-Specific Method: This calculator uses the NICHD Fetal Growth Studies equations with day-specific gestational age — the most accurate approach for classifying fetal size. Using completed-week or nearest-week rounding produces clinically significant misclassification (see Evidence tab).
Gestational Age & Estimated Fetal Weight
Enter the additional days beyond completed weeks (e.g., for 32w4d enter weeks=32, days=4)
NICHD standard developed in a racially/ethnically diverse US cohort (N=2,802)
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All 8 Methods Compared — Same EFW, Same GA
All 8 methods from Grantz et al. 2026. Highlighted row = method used for result above. Dividing line separates NICHD from Hadlock methods.
Hadlock formula vs. table discrepancy: The Hadlock 1991 printed table values cannot be reproduced using the Hadlock formula — a known error in the original paper (Roberts et al. AJOG 2025;233:331.e1-11). "Hadlock Table Interpolation" uses printed values; "Hadlock Formula" uses the equation. These produce different classifications for the same fetus.
Method
GA Used
10th %ile boundary (g)
90th %ile boundary (g)
Classification
Growth Curves — 5th, 10th, 50th, 90th Percentiles
NICHD Unified
Hadlock 1991
Orange dot appears after calculation. Toggle reference standard to compare curves.
Evidence Base for This Tool
1
Primary — Day-Specific Equations
Grantz KL, Gleason JL, Grobman WA, et al. Day-specific standard for fetal growth percentiles in the Eunice Kennedy Shriver National Institute of Child Health and Human Development Fetal Growth Studies—Singletons. Am J Obstet Gynecol 2026;(March):e75–e81.
This paper — the direct source for this tool. Provides day-specific NICHD equations and demonstrates that the completed-week method misclassifies EFWs <5th and <10th percentile in up to 50% of cases.
2
Primary — NICHD Unified Standard
Grantz KL, Grewal J, Kim S, et al. Unified standard for fetal growth: the Eunice Kennedy Shriver National Institute of Child Health and Human Development Fetal Growth Studies. Am J Obstet Gynecol 2022;226:576–87.e2.
Source of the 2022 unified NICHD singleton growth reference. Developed from 2,802 participants across 12 US clinical sites (2009–2013); more representative of current US obstetric population than Hadlock reference.
3
Comparator
Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology 1991;181:129–33.
Widely-used 1991 reference. The published table cannot be reproduced using the formula provided in the paper — a discrepancy that causes additional classification errors beyond those from week-rounding alone.
4
Validation
Haas DM, Parker CB, Wing DA, et al. A description of the methods of the Nulliparous Pregnancy Outcomes Study: monitoring mothers-to-be (nuMoM2b). Am J Obstet Gynecol 2015;212:539.e1–24.
nuMoM2b cohort (N=7,387) used for external validation of classification accuracy across gestational age methods. Results were consistent with the main NICHD analysis.
5
Guideline
Society for Maternal-Fetal Medicine (SMFM), Martins JG, Biggio JR, Abuhamad A. Society for maternal-fetal medicine consult series #52: diagnosis and management of fetal growth restriction. Am J Obstet Gynecol 2020;223:B2–17.
SMFM guideline establishing clinical thresholds: <10th percentile = small for gestational age; <5th percentile = severely small; >90th percentile = large for gestational age.
Key Findings: Classification Accuracy by Method (32–36 wks, N=2,139)
Expected by definition: 5% of fetuses <5th percentile, 10% <10th percentile, 10% >90th percentile. Deviations from these targets indicate misclassification.
Method
EFW <5th %ile
EFW <10th %ile
EFW >90th %ile
Accuracy
NICHD Day-specific ✓
5.3%
10.4%
10.3%
Best
NICHD Interpolation
5.4%
10.4%
10.1%
Near-optimal
NICHD Nearest week
4.2%
8.8%
10.9%
Moderate error
NICHD Completed week
3.0%
6.2%
17.3%
Worst
Hadlock Day-specific
9.8%
17.2%
7.1%
Overclassifies SGA
Hadlock Interpolation
4.5%
11.3%
6.9%
Underclassifies LGA
Hadlock Completed week
6.2%
11.4%
12.8%
Overestimates LGA
Hadlock Nearest week
8.9%
15.2%
8.0%
Overclassifies SGA
Clinical implication: Using the NICHD completed-week method at 32+6 weeks, fetuses with EFW 1,609–1,759 g (a 151 g range) would be misclassified as normally grown when they are truly below the 10th percentile. This has direct consequences for surveillance and delivery planning.