Hadlock Fetal Weight Calculator — Estimated Fetal Weight (EFW)

How to Use This Calculator

1. Enter BPD, HC, AC, and FL in millimeters from the ultrasound report.
2. EFW in grams and lbs/oz appears instantly using the Hadlock III formula.
3. Use the Growth Assessment tab to estimate percentile and SGA/LGA category.
4. Professional tier: add gestational age and prior scan EFW for growth rate calculation.

Formula

Example

Frequently Asked Questions

• What is the Hadlock formula and how is EFW calculated? ▼
  The Hadlock formula is the most widely used method for estimating fetal weight from ultrasound biometric measurements. Published by Frank Hadlock and colleagues at the University of Texas Health Science Center in 1985, it was derived from regression analysis of 167 fetuses with known birth weights measured within 48 hours of delivery. The Hadlock III formula uses four biometric parameters: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), all measured in centimeters. The equation is: log₁₀(EFW) = 1.3596 − 0.00386 × AC × FL + 0.0064 × HC + 0.00061 × BPD × AC + 0.0424 × AC + 0.174 × FL. The result is the base-10 logarithm of the weight in grams, so the EFW = 10 raised to that power. The formula has a reported mean absolute percentage error of approximately 7–8%, meaning a fetus estimated at 3,000 g has a 95% confidence interval of roughly 2,400–3,600 g (±20%). The abdominal circumference is the single most important parameter in Hadlock formula variations and is particularly sensitive to fetal nutrition and liver glycogen stores, making it the key measurement for detecting growth restriction. Multiple Hadlock formula variants exist (using 2–4 parameters); the 4-parameter formula (Hadlock III) is the most accurate and is standard in modern obstetric ultrasound practice.
• How accurate is ultrasound fetal weight estimation? ▼
  Ultrasound estimated fetal weight (EFW) has inherent limitations that clinicians must understand to avoid over-reliance on a single measurement. The systematic error of the Hadlock formula is approximately 7–8% mean absolute percentage error (MAPE), which translates to ±14–16% at 2 standard deviations. For a 3,500 g term fetus, this means the true birth weight could range from approximately 2,975 to 4,025 grams — a 1 kg range. Several factors affect accuracy: image quality and operator technique account for substantial variability, with BPD measurement particularly sensitive to fetal position and head shape (dolichocephaly can cause BPD underestimation — head circumference is more reliable in these cases); maternal body habitus and oligohydramnios reduce image quality; fetal anomalies (particularly hydrocephalus, microcephaly, skeletal dysplasias) may introduce systematic errors. The accuracy of EFW in detecting macrosomia (birth weight >4,000 g) is modest — sensitivity approximately 60–70% at standard cutoffs. The ACOG Committee Opinion on macrosomia notes that ultrasound is not superior to clinical estimation by Leopold maneuvers in experienced hands. Serial measurements every 2–3 weeks are more valuable than a single estimate because growth trajectory (whether the fetus is on a consistent percentile or falling) provides more clinical information than any individual EFW value.
• What do the ultrasound biometric measurements mean? ▼
  Four standard biometric parameters are used in the Hadlock formula. The Biparietal Diameter (BPD) is the transverse diameter of the fetal skull measured at the level of the thalami and cavum septum pellucidum on a transverse axial plane, from the outer edge of the near skull to the inner edge of the far skull (outer-to-inner convention in most protocols). BPD correlates well with gestational age in the second trimester. The Head Circumference (HC) is measured on the same axial plane as BPD and is more reliable than BPD when head shape is abnormal (dolichocephaly, brachycephaly). HC uses the ellipse method on the outer edge of the calvarium. The Abdominal Circumference (AC) is the most important parameter for growth assessment. It is measured in a transverse plane at the level of the stomach, umbilical vein within the liver, and portal sinus (the junction of the left and right portal veins). AC reflects liver size and subcutaneous fat — both sensitive markers of fetal nutrition and insulin action. In growth-restricted fetuses, AC is disproportionately reduced. In macrosomic fetuses (e.g., diabetic mothers), AC is disproportionately enlarged. The Femur Length (FL) is the length of the ossified diaphysis of the femur, measured on a plane perpendicular to the long axis, excluding the distal femoral epiphysis.
• What is small for gestational age (SGA) and how is it managed? ▼
  Small for gestational age (SGA) is defined as an estimated fetal weight (EFW) or birth weight below the 10th percentile for gestational age. Severe SGA is below the 3rd percentile. SGA is caused by two broad mechanisms: constitutional smallness (healthy but genetically small fetus — these fetuses have normal Doppler and grow along their low percentile without evidence of compromise); and intrauterine growth restriction (IUGR or FGR — fetal growth restriction due to inadequate placental transfer of oxygen and nutrients, often from placental insufficiency). Distinguishing constitutional SGA from FGR is critical: FGR fetuses are at increased risk of hypoxia, stillbirth, and long-term neurodevelopmental sequelae. The Doppler velocimetry of the umbilical artery is the key tool: elevated umbilical artery pulsatility index (UA PI >95th percentile), absent end-diastolic flow, or reversed end-diastolic flow represent progressively severe fetal compromise. Management of FGR depends on gestational age and severity: serial growth scans every 2–3 weeks, twice-weekly biophysical profiles and NSTs, Doppler monitoring of umbilical and middle cerebral arteries, and planned early delivery when risks of stillbirth outweigh prematurity risks. The TRUFFLE trial and PORTO trial provide current evidence for surveillance and delivery timing in FGR.
• What are the other fetal weight estimation formulas and why use Hadlock? ▼
  Numerous fetal weight estimation formulas have been published, each developed from different populations and using different parameter combinations. Major alternatives to Hadlock include: the Shepard formula (1982) using BPD and AC only — less accurate than 4-parameter formulas; the Warsof formula using BPD and AC; the Intergrowth-21st formula, developed from an international multicenter study of healthy pregnancies in diverse populations (2014–2017), which provides WHO-endorsed growth standards considered more representative of optimal fetal growth globally than older reference populations; the Gardosi formula used in the UK GROW customized growth chart software, which adjusts for maternal height, weight, parity, and ethnicity to produce individualized growth standards. Hadlock III remains the most widely used formula globally because it was derived from a large well-documented dataset, uses four parameters with complementary biological information, has extensive validation across multiple populations, and is built into nearly all major ultrasound machines and obstetric software. Its main limitation is that it was derived from a predominantly white US population in the 1980s, and may systematically underestimate or overestimate weight in certain ethnic groups. Population-specific formulas (e.g., for Asian, African, or South Asian populations) show better accuracy within their target groups. The clinician should use whichever formula is built into the institutional ultrasound report and growth chart system.

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