Hadlock Formula Fetal Weight Calculator

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Hadlock Formula Fetal Weight Calculator | Professional Ultrasound Tools :root { –primary: #004a99; –secondary: #003366; –success: #28a745; –light: #f8f9fa; –border: #dee2e6; –text: #333; –shadow: 0 4px 6px rgba(0,0,0,0.1); } body { font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif; line-height: 1.6; color: var(–text); background-color: var(–light); margin: 0; padding: 0; } .container { max-width: 960px; margin: 0 auto; padding: 20px; } header, footer { text-align: center; padding: 40px 20px; background: white; margin-bottom: 20px; border-bottom: 1px solid var(–border); } h1 { color: var(–primary); margin-bottom: 10px; font-size: 2.2rem; } h2 { color: var(–secondary); margin-top: 40px; border-bottom: 2px solid var(–primary); padding-bottom: 10px; } h3 { color: var(–primary); margin-top: 25px; } .calc-wrapper { background: white; padding: 30px; border-radius: 8px; box-shadow: var(–shadow); margin-bottom: 40px; border: 1px solid var(–border); } .input-grid { display: block; /* Enforcing single column as requested */ } .input-group { margin-bottom: 20px; } label { display: block; font-weight: 600; margin-bottom: 8px; color: var(–secondary); } input[type="number"], select { width: 100%; padding: 12px; border: 1px solid #ccc; border-radius: 4px; font-size: 16px; box-sizing: border-box; /* Fix padding issues */ } input[type="number"]:focus { border-color: var(–primary); outline: none; box-shadow: 0 0 0 3px rgba(0, 74, 153, 0.1); } .helper-text { font-size: 12px; color: #666; margin-top: 4px; } .error-msg { color: #dc3545; font-size: 12px; margin-top: 4px; display: none; } .btn-group { margin-top: 30px; display: flex; gap: 15px; flex-wrap: wrap; } button { padding: 12px 24px; border: none; border-radius: 4px; font-size: 16px; font-weight: 600; cursor: pointer; transition: background 0.2s; } .btn-primary { background-color: var(–primary); color: white; flex: 2; } .btn-primary:hover { background-color: var(–secondary); } .btn-secondary { background-color: #6c757d; color: white; flex: 1; } .btn-outline { background-color: white; border: 2px solid var(–primary); color: var(–primary); flex: 1; } .results-section { margin-top: 40px; padding-top: 30px; border-top: 1px solid var(–border); display: none; /* Hidden by default */ } .highlight-result { background-color: #e8f0fe; padding: 20px; border-radius: 6px; text-align: center; border: 1px solid #b3d7ff; margin-bottom: 25px; } .highlight-result .label { font-size: 14px; text-transform: uppercase; letter-spacing: 1px; color: var(–secondary); } .highlight-result .value { font-size: 36px; font-weight: 700; color: var(–primary); margin: 10px 0; } .highlight-result .sub-value { font-size: 18px; color: #555; } table { width: 100%; border-collapse: collapse; margin: 20px 0; background: white; } th, td { padding: 12px; text-align: left; border-bottom: 1px solid var(–border); } th { background-color: #f1f3f5; color: var(–secondary); font-weight: 600; } .chart-container { margin: 30px 0; background: white; border: 1px solid var(–border); padding: 15px; border-radius: 6px; text-align: center; } canvas { max-width: 100%; height: auto; } .article-content { background: white; padding: 40px; border-radius: 8px; box-shadow: var(–shadow); border: 1px solid var(–border); } .toc { background: #f8f9fa; padding: 20px; border-radius: 4px; margin-bottom: 30px; border: 1px solid #e9ecef; } .toc ul { list-style: none; padding: 0; } .toc li { margin-bottom: 8px; } .toc a { color: var(–primary); text-decoration: none; } .toc a:hover { text-decoration: underline; } .faq-item { margin-bottom: 20px; } .faq-question { font-weight: 700; color: var(–secondary); cursor: pointer; } .variable-table td { font-size: 0.95rem; } @media (max-width: 600px) { .btn-group { flex-direction: column; } .article-content { padding: 20px; } }

Hadlock Formula Fetal Weight Calculator

Professional Ultrasound Biometry & EFW Tool

Used to compare results against averages (14-42 weeks).
Please enter a valid week (14-42).
Distance between the parietal bones of the skull.
Please enter a valid BPD in mm.
Circumference of the fetal head.
Please enter a valid HC in mm.
Circumference of the fetal abdomen.
Please enter a valid AC in mm.
Length of the thigh bone.
Please enter a valid FL in mm.
Estimated Fetal Weight (EFW)
0 g
0 lbs 0 oz

Comparison: Calculated Weight vs. 50th Percentile for GA

Detailed Analysis

Parameter Input / Value Unit

Methodology: Calculations performed using the Hadlock IV formula (BPD, HC, AC, FL). Note: Ultrasound weight estimates typically have a margin of error of ±15%.

Table of Contents

What is the Hadlock Formula Fetal Weight Calculator?

The Hadlock formula fetal weight calculator is the gold standard method used in obstetrics and sonography to estimate the weight of a fetus during pregnancy. Developed by Dr. Frank Hadlock and colleagues in the 1980s, these mathematical models utilize biometric measurements obtained via ultrasound to predict Estimated Fetal Weight (EFW).

While there are several variations (Hadlock I, II, III, IV), the most robust version—used by this calculator—incorporates four key parameters: Head Circumference (HC), Biparietal Diameter (BPD), Abdominal Circumference (AC), and Femur Length (FL). This 4-parameter approach is widely considered to have the lowest random error compared to formulas using fewer variables.

Medical professionals use the Hadlock formula to track fetal growth patterns, identify potential growth restrictions (IUGR), or detect macrosomia (excessive growth), assisting in crucial clinical decisions regarding delivery timing and method.

Hadlock Formula and Mathematical Explanation

The calculation of fetal weight is based on a logarithmic regression equation derived from studying thousands of pregnancies. The inputs are first converted to centimeters (cm) for the calculation, and the result is transformed from a logarithmic scale back to grams.

The Hadlock IV Equation

The mathematical formula used to calculate the common log ($\log_{10}$) of the weight is:

Log10(EFW) = 1.3596 + (0.00061 × BPD × AC) + (0.0424 × AC) + (0.174 × FL) + (0.0064 × HC) – (0.00386 × AC × FL)

Once the log value is determined, the final weight in grams is calculated as:

EFW = 10Result

Variables Table

Variable Definition Unit Used in Formula Typical Range (Term)
BPD Biparietal Diameter (width of head) cm 9.0 – 10.0 cm
HC Head Circumference cm 32.0 – 35.0 cm
AC Abdominal Circumference cm 31.0 – 35.0 cm
FL Femur Length (thigh bone) cm 7.0 – 8.0 cm

Practical Examples (Real-World Use Cases)

Example 1: 34 Weeks Gestation

A sonographer measures a fetus at 34 weeks. The measurements are as follows:

  • BPD: 85 mm (8.5 cm)
  • HC: 310 mm (31.0 cm)
  • AC: 300 mm (30.0 cm)
  • FL: 65 mm (6.5 cm)

Calculation Process:
Using the Hadlock formula, the Log10(EFW) is calculated based on these centimeters. The resulting log value approximates 3.365.

Result:
The estimated weight is roughly 2,317 grams (approx 5 lbs 2 oz). This places the fetus near the 50th percentile for 34 weeks, indicating normal growth.

Example 2: Macrosomia Suspected (Term Baby)

At 40 weeks, a patient presents with a large fundal height. Ultrasound biometry shows:

  • BPD: 98 mm
  • HC: 350 mm
  • AC: 380 mm (Larger than average)
  • FL: 76 mm

Result:
The calculated EFW is approximately 4,150 grams (9 lbs 2 oz). An AC measurement significantly larger than the head measurements often drives the weight estimate up, alerting the clinician to potential macrosomia.

How to Use This Hadlock Formula Fetal Weight Calculator

Using this calculator is straightforward and mirrors the data entry process on a standard ultrasound machine.

  1. Enter Gestational Age (Optional but Recommended): Input the current weeks of pregnancy. This allows the tool to compare the estimated weight against the average for that gestational age on the chart.
  2. Input Biometric Data: Enter the BPD, HC, AC, and FL in millimeters (mm). Ensure you use standard standardized planes of section for accuracy.
  3. Click Calculate: The tool will process the Hadlock IV algorithm instantly.
  4. Review Results:
    • Primary Result: Shows weight in grams and pounds/ounces.
    • Chart: Visualizes the calculated weight relative to the 50th percentile growth curve.
    • Table: Summarizes the inputs used for documentation.

Key Factors That Affect Fetal Weight Results

While the Hadlock formula fetal weight calculator is highly accurate, several factors can influence the precision of the estimate.

1. Measurement Error (Inter-observer Variability)

The formula is only as good as the measurements entered. Slight deviations in caliper placement—especially for the Abdominal Circumference (AC)—can swing weight estimates by 10-15%. The AC is the most variable parameter because the fetal abdomen is soft and compressible.

2. Fetal Position

If the fetus is in a difficult position (e.g., deep in the pelvis or posterior), obtaining a clear cross-section of the head (BPD/HC) or abdomen (AC) becomes challenging, leading to suboptimal measurements.

3. Maternal Obesity

High maternal BMI can attenuate ultrasound waves, degrading image quality. This "haziness" makes precise edge detection of bone and tissue difficult, potentially reducing the accuracy of the Hadlock calculation.

4. Amniotic Fluid Volume

Oligohydramnios (low fluid) can crowd the fetus, compressing the abdomen and making AC measurement difficult. Conversely, Polyhydramnios (excess fluid) might allow the fetus to move excessively, making it hard to capture a still image for measurement.

5. Fetal Head Shape (Dolichocephaly vs. Brachycephaly)

The BPD can be unreliable if the head shape is unusual. A flattened head (dolichocephaly) yields a smaller BPD, underestimating weight. In these cases, the Head Circumference (HC) is a more reliable predictor, which is why the 4-parameter Hadlock formula (which includes HC) is superior to BPD-only formulas.

6. Biological Variation

The Hadlock formula assumes standard density of fetal tissue. However, fetuses of diabetic mothers may have different body composition (more subcutaneous fat), which might not be fully reflected in linear bone measurements like FL.

Frequently Asked Questions (FAQ)

How accurate is the Hadlock formula?

The Hadlock formula typically has a margin of error of ±15%. This means a baby estimated at 3,000g could realistically weigh between 2,550g and 3,450g at birth.

Why is Abdominal Circumference (AC) so important?

AC is the single most sensitive indicator of fetal growth disorders. It reflects the size of the liver and subcutaneous fat, which are the first areas affected by poor nutrition or placental insufficiency.

Can I use this calculator for twins?

Yes, the biometric formulas apply to each twin individually. However, twin growth patterns often diverge from singletons after 30-32 weeks.

What is considered a normal fetal weight?

Normal weight is generally defined as falling between the 10th and 90th percentiles for gestational age. Weights below the 10th percentile may indicate Small for Gestational Age (SGA), while above the 90th may indicate Large for Gestational Age (LGA).

Does the Hadlock formula work for all ethnicities?

Hadlock data was originally derived from a predominantly white US population. While widely used globally, some researchers suggest customized growth charts may be more accurate for specific ethnic groups.

What is the difference between Hadlock I, II, III, and IV?

These refer to different combinations of biometric parameters. Hadlock IV (BPD, HC, AC, FL) uses the most data points and is generally considered the most robust against measurement errors in any single parameter.

Is 2D ultrasound weight estimation different from 3D?

While 3D volumetric ultrasound exists, 2D biometry using the Hadlock formula remains the clinical standard due to its established reliability and ease of use.

When should I rely on clinical exams vs ultrasound?

In the third trimester, clinical palpation (Leopold's maneuvers) can estimate weight but is subjective. Ultrasound provides objective data, though accuracy decreases as the fetus becomes larger and fluid decreases near term.

Related Tools and Internal Resources

Enhance your obstetrical toolkit with these related calculators and guides:

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Disclaimer: This Hadlock formula fetal weight calculator is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment.

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Validation var hasError = false; // GA optional but if present must be reasonable if (document.getElementById('gaWeeks').value !== "" && (isNaN(gaWeeks) || gaWeeks 42)) { showError('gaError'); hasError = true; } else { hideError('gaError'); } if (isNaN(bpd) || bpd 110) { showError('bpdError'); hasError = true; } else { hideError('bpdError'); } if (isNaN(hc) || hc 400) { showError('hcError'); hasError = true; } else { hideError('hcError'); } if (isNaN(ac) || ac 400) { showError('acError'); hasError = true; } else { hideError('acError'); } if (isNaN(fl) || fl 90) { showError('flError'); hasError = true; } else { hideError('flError'); } if (hasError) return; // 3. Calculation – Hadlock IV // Formula requires CM. Inputs are MM. var bpdCm = bpd / 10; var hcCm = hc / 10; var acCm = ac / 10; var flCm = fl / 10; // Log10(EFW) = 1.3596 + 0.00061(BPD*AC) + 0.0424(AC) + 0.174(FL) + 0.0064(HC) – 0.00386(AC*FL) var term1 = 1.3596; var term2 = 0.00061 * bpdCm * acCm; var term3 = 0.0424 * acCm; var term4 = 0.174 * flCm; var term5 = 0.0064 * hcCm; var term6 = 0.00386 * acCm * flCm; var logWeight = term1 + term2 + term3 + term4 + term5 – term6; var weightGrams = Math.pow(10, logWeight); // Convert to lbs/oz var weightLbsTotal = weightGrams * 0.00220462; var weightLbs = Math.floor(weightLbsTotal); var weightOz = Math.round((weightLbsTotal – weightLbs) * 16); // 4. Update UI setHtml('resultMain', Math.round(weightGrams).toLocaleString() + " g"); setHtml('resultSub', weightLbs + " lbs " + weightOz + " oz"); // Update Table var tableHtml = "; tableHtml += 'Gestational Age' + (gaWeeks ? gaWeeks + ' Weeks' : 'Not Specified') + 'Weeks'; tableHtml += 'BPD' + bpd + 'mm'; tableHtml += 'Head Circumference' + hc + 'mm'; tableHtml += 'Abdominal Circumference' + ac + 'mm'; tableHtml += 'Femur Length' + fl + 'mm'; tableHtml += 'Calculated EFW' + Math.round(weightGrams) + 'g'; setHtml('resultTableBody', tableHtml); document.getElementById('results').style.display = 'block'; // 5. Draw Chart drawChart(gaWeeks, Math.round(weightGrams)); } function resetCalculator() { document.getElementById('gaWeeks').value = "; document.getElementById('bpd').value = "; document.getElementById('hc').value = "; document.getElementById('ac').value = "; document.getElementById('fl').value = "; hideError('gaError'); hideError('bpdError'); hideError('hcError'); hideError('acError'); hideError('flError'); document.getElementById('results').style.display = 'none'; } function copyResults() { var efw = document.getElementById('resultMain').innerText; var efwLbs = document.getElementById('resultSub').innerText; var bpd = document.getElementById('bpd').value; var ac = document.getElementById('ac').value; var fl = document.getElementById('fl').value; var hc = document.getElementById('hc').value; var text = "Hadlock Formula EFW Results:\n"; text += "Estimated Weight: " + efw + " (" + efwLbs + ")\n"; text += "Inputs – BPD: " + bpd + "mm, HC: " + hc + "mm, AC: " + ac + "mm, FL: " + fl + "mm"; navigator.clipboard.writeText(text).then(function() { var btn = document.querySelector('.btn-outline'); var originalText = btn.innerText; btn.innerText = "Copied!"; setTimeout(function() { btn.innerText = originalText; }, 2000); }); } // Canvas Chart Logic function drawChart(ga, userWeight) { var canvas = document.getElementById('growthChart'); var ctx = canvas.getContext('2d'); var width = canvas.width = canvas.parentElement.offsetWidth; var height = canvas.height; // Clear canvas ctx.clearRect(0, 0, width, height); // Define simple Hadlock 50th percentile lookup (approximate grams) // Weeks 14 to 42 var standardWeights = { 14: 45, 15: 70, 16: 100, 17: 140, 18: 190, 19: 240, 20: 300, 21: 360, 22: 430, 23: 500, 24: 600, 25: 660, 26: 760, 27: 875, 28: 1000, 29: 1150, 30: 1320, 31: 1500, 32: 1700, 33: 1900, 34: 2150, 35: 2400, 36: 2650, 37: 2900, 38: 3100, 39: 3300, 40: 3500, 41: 3700, 42: 3900 }; var labels = ["50th Percentile (Avg)", "Your Result"]; var avgWeight = 0; if (ga && standardWeights[ga]) { avgWeight = standardWeights[ga]; } else { // If no GA, we can't show avg, just show the bar for user avgWeight = 0; labels = ["Your Result"]; } var dataPoints = []; if (avgWeight > 0) dataPoints.push(avgWeight); dataPoints.push(userWeight); var maxVal = Math.max(avgWeight, userWeight) * 1.2; var barWidth = 60; var spacing = (width – (dataPoints.length * barWidth)) / (dataPoints.length + 1); // Draw Axes ctx.beginPath(); ctx.strokeStyle = '#ccc'; ctx.moveTo(40, 20); ctx.lineTo(40, height – 30); ctx.lineTo(width – 20, height – 30); ctx.stroke(); // Draw Bars for (var i = 0; i < dataPoints.length; i++) { var val = dataPoints[i]; var barHeight = (val / maxVal) * (height – 60); var x = 40 + spacing + (i * (barWidth + spacing)); var y = height – 30 – barHeight; ctx.fillStyle = (dataPoints.length === 2 && i === 1) ? '#004a99' : '#b3d7ff'; // Highlight user result if (dataPoints.length === 1) ctx.fillStyle = '#004a99'; ctx.fillRect(x, y, barWidth, barHeight); // Label value ctx.fillStyle = '#333'; ctx.font = 'bold 12px sans-serif'; ctx.textAlign = 'center'; ctx.fillText(val + "g", x + barWidth/2, y – 5); // Label Category ctx.font = '12px sans-serif'; ctx.fillText(labels[i], x + barWidth/2, height – 10); } // Y Axis Label ctx.save(); ctx.translate(15, height / 2); ctx.rotate(-Math.PI / 2); ctx.textAlign = 'center'; ctx.fillText("Weight (g)", 0, 0); ctx.restore(); }

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