Fetal Weight Calculator – Estimated Fetal Weight (EFW) & Percentiles
:root {
–primary-color: #004a99;
–primary-hover: #003377;
–success-color: #28a745;
–bg-color: #f8f9fa;
–text-color: #333;
–border-color: #ddd;
–shadow: 0 4px 6px rgba(0,0,0,0.1);
}
* {
box-sizing: border-box;
margin: 0;
padding: 0;
}
body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
background-color: var(–bg-color);
color: var(–text-color);
line-height: 1.6;
}
.container {
max-width: 960px;
margin: 0 auto;
padding: 20px;
}
header {
text-align: center;
margin-bottom: 40px;
padding: 20px 0;
border-bottom: 2px solid var(–primary-color);
}
h1 {
color: var(–primary-color);
font-size: 2.5rem;
margin-bottom: 10px;
}
h2, h3 {
color: var(–primary-color);
margin-top: 30px;
margin-bottom: 15px;
}
.loan-calc-container {
background: #fff;
padding: 30px;
border-radius: 8px;
box-shadow: var(–shadow);
margin-bottom: 40px;
}
.input-section {
margin-bottom: 30px;
}
.input-group {
margin-bottom: 20px;
}
.input-group label {
display: block;
font-weight: 600;
margin-bottom: 5px;
color: #555;
}
.input-group input, .input-group select {
width: 100%;
padding: 12px;
border: 1px solid var(–border-color);
border-radius: 4px;
font-size: 16px;
transition: border-color 0.3s;
}
.input-group input:focus {
border-color: var(–primary-color);
outline: none;
}
.helper-text {
font-size: 0.85rem;
color: #666;
margin-top: 4px;
}
.error-msg {
color: #dc3545;
font-size: 0.85rem;
margin-top: 4px;
display: none;
}
.btn-group {
display: flex;
gap: 15px;
margin-top: 20px;
}
button {
padding: 12px 24px;
border: none;
border-radius: 4px;
font-size: 16px;
font-weight: 600;
cursor: pointer;
transition: background 0.3s;
}
.btn-reset {
background-color: #6c757d;
color: white;
}
.btn-copy {
background-color: var(–primary-color);
color: white;
}
button:hover {
opacity: 0.9;
}
.results-section {
background-color: #f1f8ff;
padding: 25px;
border-radius: 6px;
border-left: 5px solid var(–primary-color);
margin-top: 30px;
}
.main-result {
text-align: center;
margin-bottom: 25px;
padding-bottom: 20px;
border-bottom: 1px solid #dae0e5;
}
.main-result-label {
font-size: 1.1rem;
color: #555;
margin-bottom: 5px;
}
.main-result-value {
font-size: 2.5rem;
font-weight: 700;
color: var(–primary-color);
}
.intermediate-results {
display: flex;
justify-content: space-between;
flex-wrap: wrap;
gap: 20px;
}
.int-res-item {
flex: 1;
min-width: 140px;
text-align: center;
background: white;
padding: 15px;
border-radius: 4px;
box-shadow: 0 2px 4px rgba(0,0,0,0.05);
}
.int-res-label {
font-size: 0.9rem;
color: #666;
margin-bottom: 5px;
}
.int-res-value {
font-size: 1.25rem;
font-weight: 600;
color: #333;
}
.chart-container {
margin-top: 40px;
position: relative;
background: white;
padding: 15px;
border-radius: 8px;
border: 1px solid var(–border-color);
}
canvas {
width: 100%;
height: 350px;
display: block;
}
table {
width: 100%;
border-collapse: collapse;
margin-top: 20px;
background: white;
}
th, td {
padding: 12px;
text-align: left;
border-bottom: 1px solid var(–border-color);
}
th {
background-color: var(–primary-color);
color: white;
}
tr:nth-child(even) {
background-color: #f8f9fa;
}
.article-content {
margin-top: 60px;
background: white;
padding: 40px;
border-radius: 8px;
box-shadow: var(–shadow);
}
.article-content p {
margin-bottom: 1.2rem;
}
.article-content ul, .article-content ol {
margin-bottom: 1.5rem;
padding-left: 25px;
}
.article-content li {
margin-bottom: 0.5rem;
}
.faq-item {
margin-bottom: 20px;
}
.faq-question {
font-weight: 700;
color: var(–primary-color);
margin-bottom: 8px;
}
.related-links {
margin-top: 30px;
padding: 20px;
background: #f1f8ff;
border-radius: 6px;
}
.related-links a {
color: var(–primary-color);
text-decoration: none;
font-weight: 600;
}
.related-links a:hover {
text-decoration: underline;
}
.caption {
text-align: center;
font-size: 0.9rem;
color: #666;
margin-top: 10px;
font-style: italic;
}
/* Responsive adjustments */
@media (max-width: 600px) {
h1 { font-size: 2rem; }
.intermediate-results { flex-direction: column; }
.btn-group { flex-direction: column; }
}
Estimated Fetal Weight (EFW)
1500 g
Method Used: Hadlock IV Formula (Log10 EFW)
Figure 1: Estimated Fetal Weight vs. Standard Growth Curves (10th, 50th, 90th percentiles).
Growth Projection Table
Based on current inputs and standard growth velocity.
| Gestational Age |
Est. Weight (g) |
Est. Weight (lbs) |
Growth Status |
Table 1: Projected fetal weight for surrounding weeks assuming constant growth percentile.
What is a Fetal Weight Calculator?
A fetal weight calculator is a specialized medical tool used by sonographers, obstetricians, and expectant parents to estimate the weight of a developing baby during pregnancy. Unlike simple date-based trackers, a true fetal weight calculator uses biometric measurements obtained from an ultrasound scan to calculate the Estimated Fetal Weight (EFW).
Monitoring fetal weight is critical for identifying potential health issues such as Intrauterine Growth Restriction (IUGR) or Macrosomia (excessive growth). By inputting key bone and circumference measurements, this calculator applies mathematically validated algorithms to predict the baby's mass.
This tool is most accurate when used in the second and third trimesters (after 20 weeks). It serves as a vital bridge between the visual data seen on an ultrasound screen and actionable medical insights regarding fetal development.
Fetal Weight Calculator Formula and Mathematical Explanation
The most widely accepted method for estimating fetal weight is the Hadlock Formula. Specifically, this calculator uses the Hadlock IV methodology, which incorporates four key biometric parameters to minimize error.
The mathematical regression equation is logarithmic. The formula determines the Log10 of the weight in grams based on measurements converted to centimeters.
Log10(EFW) = 1.3596 – 0.00386(AC × FL) + 0.0064(HC) + 0.00061(BPD × AC) + 0.0424(AC) + 0.174(FL)
Where EFW is the Estimated Fetal Weight. The variables are defined as follows:
| Variable |
Meaning |
Measurement Unit |
Typical Range (30w) |
| AC |
Abdominal Circumference |
Centimeters (cm) |
24.0 – 28.0 cm |
| FL |
Femur Length |
Centimeters (cm) |
5.4 – 6.0 cm |
| HC |
Head Circumference |
Centimeters (cm) |
27.0 – 29.0 cm |
| BPD |
Biparietal Diameter |
Centimeters (cm) |
7.5 – 8.2 cm |
Table 2: Variables used in the fetal weight calculator formula.
Practical Examples (Real-World Use Cases)
Example 1: The 30-Week Checkup
Sarah is 30 weeks pregnant. Her ultrasound technician measures the baby and provides the following data: BPD: 78mm, HC: 280mm, AC: 260mm, FL: 56mm.
Using the fetal weight calculator:
- Input: Weeks: 30, BPD: 78, HC: 280, AC: 260, FL: 56.
- Calculation: The Hadlock formula processes these geometries.
- Result: ~1,500 grams (3 lbs 5 oz).
- Interpretation: This places the baby squarely in the 50th percentile, indicating perfectly average growth for this gestational age.
Example 2: Monitoring for Macrosomia
Emily is at 38 weeks. Her doctor is concerned the baby might be large (Macrosomia). Measurements are: BPD: 98mm, HC: 340mm, AC: 360mm, FL: 76mm.
- Input: Weeks: 38, BPD: 98, HC: 340, AC: 360, FL: 76.
- Result: ~3,850 grams (8 lbs 8 oz).
- Interpretation: This weight is above the 90th percentile for 38 weeks. The medical team may use this fetal weight calculator result to discuss delivery options or labor management.
How to Use This Fetal Weight Calculator
- Gather Measurements: Obtain your latest ultrasound report. You need BPD, HC, AC, and FL in millimeters.
- Enter Gestational Age: Input how many weeks pregnant you are today. This is crucial for calculating the percentile.
- Input Biometrics: Enter the four measurements into their respective fields. Ensure you use millimeters (e.g., 26cm = 260mm).
- Review Results: The calculator instantly updates. Look at the "Estimated Fetal Weight" for the raw mass and the "Growth Percentile" to see how your baby compares to peers.
- Analyze the Chart: Check the graph to see if the plotted point (your baby) lies between the curves (normal range) or outside them.
Key Factors That Affect Fetal Weight Calculator Results
Several variables influence the output and accuracy of any fetal weight calculator:
- Measurement Precision: Ultrasound is an estimation, not a scale. Poor fetal position or low amniotic fluid can make measuring AC or FL difficult, leading to errors of up to 10-15%.
- Fetal Gender: Male fetuses tend to be slightly heavier than female fetuses at the same gestational age, though most generic calculators use a unisex standard.
- Maternal Health: Factors like gestational diabetes often lead to higher AC measurements (and thus higher weight estimates), while hypertension can lead to lower weights.
- Genetics: Tall parents often have babies with longer femurs (FL). Since FL is a multiplier in the formula, this can artificially inflate the estimated weight even if the baby is lean.
- Placental Function: The placenta provides nutrients. Insufficiency here directly reduces the Abdominal Circumference (AC), which is the most weight-sensitive variable in the formula.
- Amniotic Fluid Index (AFI): High or low fluid levels can distort the ultrasound waves, slightly altering the measurement of bone lengths.
Frequently Asked Questions (FAQ)
How accurate is this fetal weight calculator?
Ultrasound-based weight estimation typically has a margin of error of +/- 15%. A baby estimated at 2000g could realistically be between 1700g and 2300g.
Why is Abdominal Circumference (AC) so important?
AC reflects the liver size and subcutaneous fat of the fetus. It is the only measurement that varies significantly with fetal nutrition, making it the best predictor of weight issues.
What is a normal percentile range?
Generally, a weight between the 10th and 90th percentile is considered appropriate for gestational age (AGA). Below 10th suggests small for gestational age (SGA), and above 90th suggests large for gestational age (LGA).
Can I use this calculator before 20 weeks?
No. Before 20 weeks, fetal weight is negligible and less clinically relevant than structural development. Hadlock formulas are optimized for the second half of pregnancy.
Does a high estimated weight mean I need a C-section?
Not necessarily. While a high result on a fetal weight calculator can indicate a large baby, clinical decisions depend on pelvic size, diabetic status, and past delivery history.
Why does the result differ from my doctor's estimate?
Your doctor may use a different formula (e.g., Shepard, Warsof) or different population charts. This tool uses the standard Hadlock IV formula.
What if my measurements are in cm?
Multiply your centimeter measurements by 10 to get millimeters for this calculator (e.g., 5.4 cm = 54 mm).
How often should I check fetal weight?
Medical professionals typically measure growth every 3 to 4 weeks. Measuring too frequently produces unreliable growth velocity data.
Related Tools and Internal Resources
// Global chart variable
var growthChartInstance = null;
// Standard weights (approximate 50th percentile) for weeks 20-42 in grams
// Source: Generic Hadlock/WHO composite for visualization
var standardWeights = {
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: 2100,
35: 2400, 36: 2600, 37: 2900, 38: 3100, 39: 3300,
40: 3500, 41: 3600, 42: 3700
};
function init() {
calculateFetalWeight();
}
// Helper: Get element by ID
function getVal(id) {
var el = document.getElementById(id);
return el ? parseFloat(el.value) : 0;
}
function calculateFetalWeight() {
// 1. Get Inputs
var ga = getVal("gestationalAge");
var bpdMm = getVal("bpdInput");
var hcMm = getVal("hcInput");
var acMm = getVal("acInput");
var flMm = getVal("flInput");
// 2. Validate Inputs
var valid = true;
if (isNaN(ga) || ga 42) {
document.getElementById("err-ga").style.display = "block";
valid = false;
} else {
document.getElementById("err-ga").style.display = "none";
}
if (isNaN(bpdMm) || bpdMm < 10) { document.getElementById("err-bpd").style.display = "block"; valid = false; } else { document.getElementById("err-bpd").style.display = "none"; }
if (isNaN(hcMm) || hcMm < 50) { document.getElementById("err-hc").style.display = "block"; valid = false; } else { document.getElementById("err-hc").style.display = "none"; }
if (isNaN(acMm) || acMm < 50) { document.getElementById("err-ac").style.display = "block"; valid = false; } else { document.getElementById("err-ac").style.display = "none"; }
if (isNaN(flMm) || flMm < 10) { document.getElementById("err-fl").style.display = "block"; valid = false; } else { document.getElementById("err-fl").style.display = "none"; }
if (!valid) return;
// 3. Convert mm to cm for formula
var acCm = acMm / 10;
var flCm = flMm / 10;
var hcCm = hcMm / 10;
var bpdCm = bpdMm / 10;
// 4. Calculate Logic (Hadlock IV)
// Log10(EFW) = 1.3596 – 0.00386(AC*FL) + 0.0064(HC) + 0.00061(BPD*AC) + 0.0424(AC) + 0.174(FL)
var logEfw = 1.3596 – (0.00386 * acCm * flCm) + (0.0064 * hcCm) + (0.00061 * bpdCm * acCm) + (0.0424 * acCm) + (0.174 * flCm);
var efwGrams = Math.pow(10, logEfw);
// 5. Update UI Results
// Main Result (Grams)
document.getElementById("mainResult").innerText = Math.round(efwGrams) + " g";
// Intermediate: Lbs/Oz
var totalOz = efwGrams * 0.035274;
var lbs = Math.floor(totalOz / 16);
var oz = Math.round(totalOz % 16);
document.getElementById("weightLbsResult").innerText = lbs + " lbs " + oz + " oz";
// Intermediate: Percentile
// Simple simplified normal distribution approximation for demo purposes
// Mean for GA is standardWeights[ga], assume 12% StdDev
var mean = standardWeights[ga] || 3500;
var sd = mean * 0.12;
var zScore = (efwGrams – mean) / sd;
// Convert Z to percentile approx
var percentile = (1 / (1 + Math.exp(-1.7 * zScore))) * 100; // Sigmoid approx
if (percentile < 1) percentile = " 99) percentile = "> 99th";
else percentile = Math.round(percentile) + "th";
document.getElementById("percentileResult").innerText = percentile;
// Intermediate: Est Week
// Find week where calculated weight matches 50th percentile
var estWeek = "N/A";
for (var w = 20; w = efwGrams) {
// simple interpolation
var prevW = w – 1;
var weightDiff = standardWeights[w] – standardWeights[prevW];
var myDiff = efwGrams – standardWeights[prevW];
var fraction = myDiff / weightDiff;
estWeek = (prevW + fraction).toFixed(1);
break;
}
}
if (efwGrams > standardWeights[42]) estWeek = "> 42";
if (efwGrams < standardWeights[20]) estWeek = "< 20";
document.getElementById("estWeekResult").innerText = estWeek + " Weeks";
// 6. Update Table
updateTable(ga, efwGrams);
// 7. Update Chart
drawChart(ga, efwGrams);
}
function updateTable(currentGa, currentWeight) {
var tbody = document.getElementById("projectionTableBody");
tbody.innerHTML = "";
// Show current week, +2 weeks, +4 weeks assuming same percentile growth
// Calc ratio of current weight to standard weight
var stdForGa = standardWeights[currentGa] || 3500;
var ratio = currentWeight / stdForGa;
var offsets = [0, 2, 4];
for (var i = 0; i 42) break;
var stdAtWeek = standardWeights[week];
var projWeight = stdAtWeek * ratio;
var totalOz = projWeight * 0.035274;
var lbs = Math.floor(totalOz / 16);
var oz = Math.round(totalOz % 16);
var status = "Projected";
if (i === 0) status = "Current";
var row = "
" +
"| " + week + " Weeks | " +
"" + Math.round(projWeight) + " g | " +
"" + lbs + " lbs " + oz + " oz | " +
"" + status + " | " +
"
";
tbody.innerHTML += row;
}
}
function resetCalculator() {
document.getElementById("gestationalAge").value = 30;
document.getElementById("bpdInput").value = 78;
document.getElementById("hcInput").value = 280;
document.getElementById("acInput").value = 260;
document.getElementById("flInput").value = 56;
calculateFetalWeight();
}
function copyResults() {
var res = document.getElementById("mainResult").innerText;
var lbs = document.getElementById("weightLbsResult").innerText;
var pct = document.getElementById("percentileResult").innerText;
var text = "Fetal Weight Calculation:\n" +
"Estimated Weight: " + res + " (" + lbs + ")\n" +
"Percentile: " + pct + "\n" +
"Inputs: " + getVal("gestationalAge") + "w, BPD " + getVal("bpdInput") + ", HC " + getVal("hcInput") + ", AC " + getVal("acInput") + ", FL " + getVal("flInput");
var temp = document.createElement("textarea");
temp.value = text;
document.body.appendChild(temp);
temp.select();
document.execCommand("copy");
document.body.removeChild(temp);
var btn = document.querySelector(".btn-copy");
var original = btn.innerText;
btn.innerText = "Copied!";
setTimeout(function(){ btn.innerText = original; }, 2000);
}
function drawChart(userWeek, userWeight) {
var canvas = document.getElementById("growthChart");
var ctx = canvas.getContext("2d");
// Handle High DPI
var dpr = window.devicePixelRatio || 1;
var rect = canvas.getBoundingClientRect();
canvas.width = rect.width * dpr;
canvas.height = rect.height * dpr;
ctx.scale(dpr, dpr);
var width = rect.width;
var height = rect.height;
// Clear
ctx.clearRect(0, 0, width, height);
// Config
var padding = {top: 20, right: 30, bottom: 40, left: 50};
var chartW = width – padding.left – padding.right;
var chartH = height – padding.top – padding.bottom;
// Ranges
var minW = 20, maxW = 42;
var minG = 0, maxG = 4500;
// Helpers
function getX(w) {
return padding.left + ((w – minW) / (maxW – minW)) * chartW;
}
function getY(g) {
return padding.top + chartH – ((g – minG) / (maxG – minG)) * chartH;
}
// Draw Axes
ctx.beginPath();
ctx.strokeStyle = "#ddd";
ctx.lineWidth = 1;
// Y Axis Grid
for(var g=0; g<=maxG; g+=500) {
var y = getY(g);
ctx.moveTo(padding.left, y);
ctx.lineTo(width – padding.right, y);
ctx.fillStyle = "#666";
ctx.font = "10px Arial";
ctx.fillText(g, 5, y + 3);
}
// X Axis Grid
for(var w=20; w<=maxW; w+=2) {
var x = getX(w);
ctx.moveTo(x, padding.top);
ctx.lineTo(x, padding.top + chartH);
ctx.fillStyle = "#666";
ctx.font = "10px Arial";
ctx.fillText(w + "w", x – 8, height – 10);
}
ctx.stroke();
// Draw Curves (10th, 50th, 90th)
function drawCurve(percentile, color) {
ctx.beginPath();
ctx.strokeStyle = color;
ctx.lineWidth = 2;
var first = true;
for(var w=20; w= 20 && userWeek <= 42) {
var px = getX(userWeek);
var py = getY(userWeight);
// Shadow/Glow
ctx.beginPath();
ctx.arc(px, py, 8, 0, 2 * Math.PI);
ctx.fillStyle = "rgba(40, 167, 69, 0.3)";
ctx.fill();
// Point
ctx.beginPath();
ctx.arc(px, py, 4, 0, 2 * Math.PI);
ctx.fillStyle = "#28a745"; // Success color
ctx.fill();
// Label
ctx.fillStyle = "#000";
ctx.font = "bold 12px Arial";
ctx.fillText("You", px + 10, py);
}
}
// Initialize on load
window.onload = init;