| Formula | Parameters | EFW |
|---|
Compare today's Hadlock 4 EFW against a previous scan to see the weekly growth rate. Serial trend matters more than any single estimate for growth restriction or macrosomia workups.
The 6 Formulas, and When to Use Each
| Formula | Parameters | Best used when |
|---|---|---|
| Hadlock 1 | AC + FL | HC and BPD are both unreliable (deeply engaged head, dolichocephaly) |
| Hadlock 2 | BPD + AC + FL | HC measurement is unavailable but head shape is normal |
| Hadlock 3 | HC + AC + FL | Standard 3-parameter estimate; among the best-performing in accuracy comparisons |
| Hadlock 4 | BPD + HC + AC + FL | All 4 measurements are available — the most widely used clinical default |
| Shepard | BPD + AC | Only head and abdomen were measured; largely superseded but still cited |
| INTERGROWTH-21st | AC + HC | Comparing against the INTERGROWTH-21st international growth standard |
Why the 6 Numbers Won't Match
Every EFW formula is a regression equation fitted to a specific study cohort. Hadlock's four formulas (1984–1985) were derived from a US population using different parameter combinations; Shepard's formula (1982) predates them and uses only BPD and AC; the INTERGROWTH-21st formula (2016) was built from an eight-country international cohort using only AC and HC.
Because each was fitted independently, a 5–10% spread between formulas on the identical measurements is completely normal — it reflects differences in the underlying study populations and modelling choices, not measurement error on your part.
Hadlock vs. INTERGROWTH-21st in Practice
Comparative studies generally find strong correlation between Hadlock and INTERGROWTH-21st estimates, with Hadlock showing a small but consistent edge in predicting actual birth weight in several third-trimester cohorts, while INTERGROWTH-21st has performed reasonably in preterm and international comparisons.
Neither formula is "wrong" — the choice often comes down to which growth reference your unit uses for percentile classification, since the formula and the reference standard should ideally be used as a matched pair.
Choosing a Formula by Clinical Situation
Head Deeply Engaged
BPD becomes unreliable late in labour. Switch to Hadlock 1 (AC+FL only) rather than forcing a poor BPD into a 4-parameter formula.
Suspected Growth Restriction
A single EFW below a threshold matters less than the trend. Use the growth-velocity section above with 2–3 week interval scans.
Gestational Diabetes / Macrosomia Screen
AC is the most sensitive single parameter for macrosomia. Hadlock 3 or 4 (which weight AC heavily) are preferred over Shepard.
International / Research Comparison
Use the INTERGROWTH-21st formula if you need results comparable to international standard cohorts rather than a US-derived reference.