What standard does this use?

It uses the INTERGROWTH-21st international estimated fetal weight standard published by Stirnemann and colleagues in 2017, which models the logarithm of EFW as a function of gestational age. It is a prescriptive international standard rather than a local reference.

What counts as SGA and LGA?

A centile below the 10th is classified small for gestational age (SGA) and above the 90th is large for gestational age (LGA). Between the 10th and 90th centiles the weight is appropriate for gestational age (AGA).

Is a single low centile a diagnosis of growth restriction?

No. A single estimate below the 10th centile flags a small fetus but does not by itself diagnose fetal growth restriction. Serial measurements showing a falling trajectory, abnormal Dopplers, and the clinical context are needed for that judgement.

How accurate is an estimated fetal weight?

Ultrasound EFW typically carries a margin of about plus or minus 10 to 15 percent, so the centile inherits that uncertainty. Use it as a screening figure alongside the trend across scans rather than a precise weight.

What gestational age range is valid?

The INTERGROWTH-21st EFW standard is defined from 22 to 40 completed weeks. Outside that window the tool will not return a centile because the published equation does not cover it.

What is the Fetal Growth Centile Calculator (INTERGROWTH-21st)?

Calculate an estimated fetal weight centile from ultrasound EFW and gestational age using the INTERGROWTH-21st international standard, with z-score, median weight, and small (SGA, under 10th) or large (LGA, over 90th) classification. It runs free in your browser on Gera Tools, with nothing uploaded.

Fetal Growth Centile Calculator (INTERGROWTH-21st)

Name: Fetal Growth Centile Calculator (INTERGROWTH-21st)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Tracking whether a fetus is growing along an expected trajectory is central to antenatal care, and the INTERGROWTH-21st project produced a prescriptive international standard for estimated fetal weight. This calculator places a single EFW on that standard, returning the centile, median, z-score, and an SGA or LGA classification.

About the INTERGROWTH-21st standard

The INTERGROWTH-21st project recruited healthy, well-nourished women with uncomplicated pregnancies across eight geographically diverse countries, then measured fetal biometry longitudinally at standardised intervals. By using populations with optimal conditions, the resulting charts describe how fetuses should grow — a prescriptive international standard — rather than how they do grow in a given population (which would be descriptive and would embed local nutritional or health inequalities).

The EFW equation was published by Stirnemann and colleagues in 2017 and is based on standard Hadlock biometric parameters (BPD, HC, AC, FL). It is the most widely endorsed international standard for gestational-age-specific weight centiles.

How it works

The INTERGROWTH-21st EFW standard models the natural log of weight as a smooth function of gestational age. The median and the standard deviation of log(EFW) are computed from the published coefficients, then:

z       = (ln(EFW_measured) − median_log_EFW) / sd_log_EFW
centile = Φ(z) × 100

where Φ is the standard normal cumulative distribution. A centile below 10 is labelled SGA, above 90 LGA, and in between AGA. The median weight shown is the 50th-centile EFW for the entered gestational age.

Illustrative centile values

At 28 weeks, the 10th centile is roughly 900–950 g, the 50th centile around 1,100 g, and the 90th centile roughly 1,300 g (for illustration — always read the actual output). A fetus measuring 850 g at 28 weeks would fall below the 10th centile and prompt further evaluation, while a fetus at 1,050 g would be around the 40th centile and within the normal range.

SGA, AGA, and LGA in clinical practice

SGA (below 10th centile) — flags a fetus that may not have reached its growth potential. Requires correlation with Doppler studies (umbilical artery, middle cerebral artery, ductus venosus) and serial biometry to distinguish constitutionally small fetuses from those with true growth restriction.
AGA (10th–90th centile) — appropriate for gestational age; growth is tracking within the expected range.
LGA (above 90th centile) — may indicate macrosomia; screen for gestational diabetes and assess pelvic adequacy when planning delivery.

Notes and limits

The standard is defined from 22 to 40 weeks, so the tool will not return a result outside that window. Ultrasound EFW carries roughly a 10–15% margin of error even with careful technique, so the centile is a screening figure, not an exact weight. A single value below the 10th centile is not a diagnosis of growth restriction; the trajectory across serial scans, Doppler studies, and the wider clinical picture decide management. This tool is an educational reference only and should not replace clinical assessment by a qualified healthcare professional.