Calculate Estimated Fetal Weight

Your comprehensive tool for prenatal growth estimation.

Typical Fetal Growth Parameters by Gestational Age

Gestational Age (Weeks)	Estimated Fetal Weight (grams)	Femur Length (cm)	Abdominal Circumference (cm)
20	300-360	3.3	16.5
24	630-760	4.6	21.7
28	1000-1200	5.5	24.5
32	1600-1900	6.3	28.0
36	2400-2800	7.0	31.5
40	3200-3600	7.5	34.5

What is Estimated Fetal Weight (EFW)?

Estimated Fetal Weight (EFW) is a medical estimation of a baby's weight while still in the womb, typically determined through ultrasound measurements. It's a crucial indicator of fetal growth and well-being. Healthcare providers use EFW to monitor whether a baby is growing appropriately for its gestational age, identifying potential concerns like Intrauterine Growth Restriction (IUGR) or macrosomia (a baby estimated to be significantly larger than average). The calculation involves measuring specific fetal anatomical structures – primarily the Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL) – and inputting these values into various standardized mathematical formulas. These formulas leverage statistical data to approximate the baby's overall size and weight. It's vital to understand that EFW is an estimation; the actual birth weight can differ by a margin of error, which can vary based on the formula used, the technician's skill, and the baby's position.

Who Should Use This EFW Calculator?

This calculator is primarily designed for healthcare professionals, including obstetricians, gynecologists, maternal-fetal medicine specialists, and sonographers, to quickly assess and track fetal growth. Pregnant individuals might also use it for informational purposes, to better understand their baby's development as discussed with their doctor. However, it should never replace professional medical advice or diagnosis. Common misconceptions include believing EFW is a perfectly accurate measurement or that it dictates the exact birth weight. It's a tool for monitoring trends and identifying potential deviations from typical growth patterns, prompting further medical evaluation when necessary.

Estimated Fetal Weight (EFW) Formula and Mathematical Explanation

There isn't one single EFW formula; numerous variations exist, each with its strengths and target gestational age ranges. A widely used and relatively simple method is the Hadlock formula, which incorporates Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). Other formulas might also include the Biparietal Diameter (BPD) or use different combinations and weightings of these measurements. For simplicity and demonstration, we'll use a generalized approach that combines several common factors, often reflecting the output of algorithms used in many ultrasound machines.

Simplified EFW Calculation Logic

Our calculator employs a multi-factor approach, often seen in modern ultrasound software. It aims to provide a robust estimate by considering key growth parameters:

• Gestational Age (GA): The primary reference point.
• Femur Length (FL): A good indicator of skeletal growth.
• Biparietal Diameter (BPD): A measure of head size.
• Abdominal Circumference (AC): Crucial for estimating fetal mass and growth, as it reflects liver size and subcutaneous fat.
• Head Circumference (HC): Another measure of head growth.

The exact algorithms used by ultrasound machines are complex and proprietary, often involving regression analysis based on large datasets. For this calculator, we synthesize a representative calculation that balances the influence of these key parameters to estimate the fetal weight (EFW) in grams.

Core Concept: The calculator estimates EFW by assigning a growth score based on the provided measurements relative to typical values for the given gestational age. A higher score generally indicates a larger estimated fetal weight.

Simplified Representation: While specific formulas vary greatly, a common approach involves calculating a "growth index" from multiple measurements (FL, BPD, AC, HC) and then using this index along with gestational age to predict weight. For example, a modified Hadlock formula is often used:

Log10(EFW) = b0 + b1*Log10(FL) + b2*Log10(HC) + b3*Log10(AC)
Where b0, b1, b2, b3 are coefficients determined by regression analysis. The calculator performs a similar, albeit simplified, statistical estimation.

Variable Explanations

Understanding the input variables is key to using this estimated fetal weight calculator effectively:

EFW Calculator Variables

Variable	Meaning	Unit	Typical Range (for reference)
Gestational Age (GA)	Number of weeks since the first day of the last menstrual period (LMP).	Weeks	16 – 42
Femur Length (FL)	Length of the baby's thigh bone, measured via ultrasound.	cm	~3.3 cm (20 wks) to ~7.5 cm (40 wks)
Biparietal Diameter (BPD)	Widest side-to-side diameter of the fetal head, measured via ultrasound.	cm	~7.2 cm (28 wks) to ~9.5 cm (40 wks)
Abdominal Circumference (AC)	Circumference of the fetal abdomen at its widest point, measured via ultrasound.	cm	~24.5 cm (28 wks) to ~34.5 cm (40 wks)
Head Circumference (HC)	Circumference of the fetal head, measured via ultrasound.	cm	~26.0 cm (28 wks) to ~35.0 cm (40 wks)

Practical Examples (Real-World Use Cases)

Example 1: Monitoring Growth in a Healthy Pregnancy

Scenario: A pregnant individual at 30 weeks gestation has an ultrasound. The measurements are:

• Gestational Age: 30 weeks
• Femur Length (FL): 5.8 cm
• Biparietal Diameter (BPD): 7.5 cm
• Abdominal Circumference (AC): 26.0 cm
• Head Circumference (HC): 27.5 cm

Calculation: Inputting these values into the calculator yields an Estimated Fetal Weight (EFW) of approximately 1450 grams (3.2 lbs).

Interpretation: At 30 weeks, the typical EFW range is roughly 1300-1600 grams. The calculated EFW of 1450 grams falls well within the expected range, indicating normal fetal growth for this gestational age. The intermediate values would show a good consistency between the skeletal, abdominal, and head measurements.

Example 2: Investigating Potential Growth Restriction

Scenario: A pregnant individual presents at 34 weeks gestation. Concerns have been raised about the baby's size. Ultrasound measurements are:

• Gestational Age: 34 weeks
• Femur Length (FL): 6.0 cm
• Biparietal Diameter (BPD): 7.8 cm
• Abdominal Circumference (AC): 25.5 cm
• Head Circumference (HC): 27.0 cm

Calculation: Using the calculator with these inputs results in an Estimated Fetal Weight (EFW) of approximately 1700 grams (3.7 lbs).

Interpretation: At 34 weeks, the typical EFW range is around 2000-2400 grams. The calculated EFW of 1700 grams is significantly lower than expected for this gestational age. The intermediate values might also show a disproportionate growth pattern, possibly indicating Intrauterine Growth Restriction (IUGR). This finding would prompt further medical investigation, such as Doppler studies to assess blood flow and fetal well-being, and potentially more frequent monitoring or consideration of earlier delivery.

How to Use This Estimated Fetal Weight Calculator

Using the Estimated Fetal Weight (EFW) calculator is straightforward. Follow these steps for accurate estimations:

1. Obtain Ultrasound Measurements: Ensure you have accurate measurements from a recent prenatal ultrasound. These typically include Gestational Age (in weeks), Femur Length (FL), Biparietal Diameter (BPD), Abdominal Circumference (AC), and Head Circumference (HC).
2. Enter Data Accurately: Input each measurement into the corresponding field on the calculator. Pay close attention to the units (usually centimeters for lengths and circumferences, weeks for gestational age).
3. Select Appropriate Formula (if applicable): While this calculator uses a consolidated approach, advanced tools might offer different formulas. Choose one appropriate for the gestational age if options are presented.
4. Initiate Calculation: Click the "Calculate EFW" button.

Reading the Results

The calculator will display:

• Primary Result (EFW): The estimated weight of the fetus in grams and pounds. This is the main output.
• Intermediate Values: These provide context, such as average growth factors or combined skeletal measurements, helping to understand the data's consistency.
• Estimated Gestational Age: Sometimes recalculating or confirming GA based on measurements can be an output.
• Formula Used: A brief explanation of the methodology.

Decision-Making Guidance

The EFW is a guide, not a definitive answer.

• Normal Range: If the EFW falls within the expected range for the gestational age (typically between the 10th and 90th percentiles), it suggests normal growth.
• Below Range (IUGR concerns): Significantly low EFW may indicate IUGR, requiring further assessment of fetal well-being, placental function, and potential interventions.
• Above Range (Macrosomia concerns): A high EFW might suggest macrosomia, which can complicate delivery and requires careful management, especially in cases of maternal diabetes.

Always discuss the results with your healthcare provider. They will interpret the EFW in the context of your overall pregnancy, medical history, and other clinical findings. The margin of error for EFW can be significant, particularly in later pregnancy.

Key Factors That Affect EFW Results

Several factors influence the accuracy and interpretation of Estimated Fetal Weight (EFW) calculations:

1. Accuracy of Ultrasound Measurements: This is paramount. Fetal position, maternal body habitus (e.g., obesity), amniotic fluid volume, and the skill of the sonographer can all affect the precision of FL, BPD, AC, and HC measurements. Even small errors can lead to significant deviations in EFW.
2. Gestational Age Accuracy: The initial dating of the pregnancy, usually based on the last menstrual period or early ultrasound, sets the benchmark. If the GA is inaccurate, the EFW will be misinterpreted, as it's compared against norms for that assumed age.
3. Fetal Growth Patterns: Not all babies follow the same growth curve. Genetic factors, individual variations, and the specific growth trajectory of the fetus play a role. Some babies are naturally smaller or larger.
4. Placental Function: The placenta provides nutrients and oxygen. Impaired placental function can lead to reduced fetal growth (IUGR), resulting in a lower EFW than expected.
5. Maternal Health Conditions: Conditions like hypertension, diabetes (pre-existing or gestational), and infections can significantly impact fetal growth and thus EFW. Maternal nutrition also plays a critical role.
6. Amniotic Fluid Volume: Significantly low (oligohydramnios) or high (polyhydramnios) amniotic fluid can affect ultrasound measurements and may be associated with fetal growth issues.
7. Fetal Anatomy: Certain fetal anomalies or syndromes can be associated with abnormal growth patterns that affect EFW.
8. Formula Selection: As mentioned, different EFW formulas exist, and each has a slightly different accuracy profile and margin of error depending on the gestational age and population it was derived from.

Frequently Asked Questions (FAQ)

Is Estimated Fetal Weight (EFW) the same as actual birth weight?

No. EFW is an estimation derived from ultrasound measurements. The actual birth weight can differ due to several factors, including the margin of error inherent in ultrasound technology and formula calculations. The typical margin of error can range from +/- 7% to 15% or more, especially later in pregnancy.

What is considered a normal EFW range?

A normal EFW is generally considered to be between the 10th and 90th percentile for the specific gestational age. Your healthcare provider will interpret your baby's EFW within this context.

What causes Intrauterine Growth Restriction (IUGR)?

IUGR occurs when a baby doesn't grow at the expected rate inside the uterus. Causes can include problems with the placenta (reduced blood flow/nutrients), maternal health conditions (like high blood pressure or diabetes), infections, genetic factors, or lifestyle factors (smoking, substance use).

What is fetal macrosomia?

Fetal macrosomia refers to a baby estimated to be significantly larger than average for gestational age, often defined as an EFW above the 90th percentile or over 4000-4500 grams (8.8-9.9 lbs). It's commonly associated with maternal diabetes but can also occur in other situations.

How often should EFW be monitored?

The frequency of EFW monitoring depends on the individual pregnancy. If growth is consistently normal, it might be checked at routine anatomy scans (around 20 weeks) and again in late pregnancy. If there are concerns about growth (IUGR or suspected macrosomia), monitoring may be more frequent, perhaps every 2-4 weeks.

Can diet affect my baby's estimated fetal weight?

Yes, maternal diet significantly impacts fetal growth. A balanced, nutrient-rich diet supports healthy development. Inadequate nutrition can contribute to IUGR, while excessive weight gain or poor dietary choices (especially in the context of diabetes) can contribute to macrosomia.

Does the calculator account for twins or multiples?

This specific calculator is designed for singleton pregnancies. Calculating EFW for twins or multiples involves additional complexities due to shared or competing placental resources and differing growth trajectories. Consult your healthcare provider for EFW estimations in multiple gestations.

What should I do if my baby's EFW is consistently low or high?

If your baby's EFW is consistently outside the normal range, your healthcare provider will likely recommend further investigations. This may include more frequent ultrasounds, Doppler studies to assess blood flow, non-stress tests (NSTs), and a discussion about potential interventions or delivery timing. It's crucial to follow their guidance.

Related Tools and Internal Resources

function validateInput(id, min, max, errorId) { var input = document.getElementById(id); var errorDiv = document.getElementById(errorId); var value = parseFloat(input.value); errorDiv.textContent = "; // Clear previous error if (isNaN(value)) { errorDiv.textContent = 'Please enter a valid number.'; return false; } if (value <= 0) { errorDiv.textContent = 'Value must be positive.'; return false; } if (min !== null && value max) { errorDiv.textContent = 'Value is too high. Maximum is ' + max + '.'; return false; } return true; } function calculateEFW() { var gestationalAge = document.getElementById('gestationalAge'); var femurLength = document.getElementById('femurLength'); var biparietalDiameter = document.getElementById('biparietalDiameter'); var abdominalCircumference = document.getElementById('abdominalCircumference'); var headCircumference = document.getElementById('headCircumference'); var gestationalAgeError = document.getElementById('gestationalAgeError'); var femurLengthError = document.getElementById('femurLengthError'); var biparietalDiameterError = document.getElementById('biparietalDiameterError'); var abdominalCircumferenceError = document.getElementById('abdominalCircumferenceError'); var headCircumferenceError = document.getElementById('headCircumferenceError'); var valid = true; valid = validateInput('gestationalAge', 16, 42, 'gestationalAgeError') && valid; valid = validateInput('femurLength', 1, 10, 'femurLengthError') && valid; // Adjusted range for typical femur length valid = validateInput('biparietalDiameter', 1, 10, 'biparietalDiameterError') && valid; // Adjusted range valid = validateInput('abdominalCircumference', 5, 50, 'abdominalCircumferenceError') && valid; // Adjusted range valid = validateInput('headCircumference', 5, 50, 'headCircumferenceError') && valid; // Adjusted range if (!valid) { document.getElementById('mainResult').querySelector('span').textContent = '–'; document.getElementById('growthFactor').textContent = '–'; document.getElementById('skeletalMeasure').textContent = '–'; document.getElementById('estimatedGA').textContent = '–'; updateChart([], []); // Clear chart if inputs are invalid return; } var ga = parseFloat(gestationalAge.value); var fl = parseFloat(femurLength.value); var bpd = parseFloat(biparietalDiameter.value); var ac = parseFloat(abdominalCircumference.value); var hc = parseFloat(headCircumference.value); // — Simplified EFW Calculation Logic — // This is a highly simplified representation of common EFW formulas. // Real-world formulas (like Hadlock, Shepard, etc.) involve logarithmic transformations and specific coefficients. // This version attempts to capture the relative influence of measurements. // Calculate growth factor based on GA and measurements relative to typical values // These are rough approximations for demonstration var typicalFL = 0.25 * ga + 1.2; // Very rough approximation var typicalAC = 0.7 * ga + 5.0; // Very rough approximation var typicalHC = 0.75 * ga + 3.0; // Very rough approximation var flFactor = fl / typicalFL; var acFactor = ac / typicalAC; var hcFactor = hc / typicalHC; // Combine factors – giving AC more weight as it's a major contributor to weight var growthIndex = (flFactor * 0.2) + (acFactor * 0.5) + (hcFactor * 0.3); document.getElementById('growthFactor').textContent = growthIndex.toFixed(2); // Combined skeletal measure (FL + BPD) for context var skeletalMeasure = fl + bpd; document.getElementById('skeletalMeasure').textContent = skeletalMeasure.toFixed(2) + ' cm'; // Simplified EFW estimation based on GA and growth index // This is a highly generalized model. Actual formulas are regression-based. var baseWeight = Math.pow(ga, 1.8) * 15; // Base weight estimate influenced by GA var adjustedWeight = baseWeight * growthIndex; // Cap the estimation at reasonable values for typical pregnancy ranges var minWeightForGA = 100; // grams var maxWeightForGA = 5000; // grams var efwGrams = Math.max(minWeightForGA, Math.min(maxWeightForGA, adjustedWeight)); efwGrams = Math.round(efwGrams); // Round to nearest gram var efbPounds = efwGrams / 453.592; document.getElementById('mainResult').querySelector('span').textContent = efwGrams + 'g (' + efbPounds.toFixed(1) + ' lbs)'; document.getElementById('estimatedGA').textContent = ga.toFixed(0) + ' weeks'; // Update Chart Data updateChartData(ga, efwGrams); } function resetCalculator() { document.getElementById('gestationalAge').value = 28; document.getElementById('femurLength').value = 5.5; document.getElementById('biparietalDiameter').value = 7.2; document.getElementById('abdominalCircumference').value = 24.5; document.getElementById('headCircumference').value = 26.0; document.getElementById('gestationalAgeError').textContent = "; document.getElementById('femurLengthError').textContent = "; document.getElementById('biparietalDiameterError').textContent = "; document.getElementById('abdominalCircumferenceError').textContent = "; document.getElementById('headCircumferenceError').textContent = "; document.getElementById('mainResult').querySelector('span').textContent = '–'; document.getElementById('growthFactor').textContent = '–'; document.getElementById('skeletalMeasure').textContent = '–'; document.getElementById('estimatedGA').textContent = '–'; updateChart([], []); // Clear chart } function copyResults() { var mainResultSpan = document.getElementById('mainResult').querySelector('span'); var growthFactorSpan = document.getElementById('growthFactor'); var skeletalMeasureSpan = document.getElementById('skeletalMeasure'); var estimatedGASpan = document.getElementById('estimatedGA'); var copyText = "Estimated Fetal Weight Calculation:\n\n"; copyText += "EFW: " + mainResultSpan.textContent + "\n"; copyText += "Average Growth Factor: " + growthFactorSpan.textContent + "\n"; copyText += "Combined Skeletal Measure: " + skeletalMeasureSpan.textContent + "\n"; copyText += "Estimated Gestational Age: " + estimatedGASpan.textContent + "\n\n"; copyText += "Disclaimer: This is an estimated value based on inputted measurements and a generalized formula. Actual birth weight may vary."; var textarea = document.createElement("textarea"); textarea.value = copyText; document.body.appendChild(textarea); textarea.select(); try { var successful = document.execCommand('copy'); var msg = successful ? 'Results copied!' : 'Copying failed'; console.log('Copy command was ' + msg); // Optionally, show a temporary message to the user var oldMsg = document.getElementById('copyMessage'); if(oldMsg) oldMsg.remove(); var msgEl = document.createElement('div'); msgEl.id = 'copyMessage'; msgEl.textContent = msg; msgEl.style.cssText = 'position:fixed; top:50%; left:50%; transform:translate(-50%,-50%); background:rgba(0,0,0,0.7); color:white; padding:10px; border-radius:5px; z-index:1000;'; document.body.appendChild(msgEl); setTimeout(function() { msgEl.remove(); }, 2000); } catch (err) { console.log('Oops, unable to copy'); } document.body.removeChild(textarea); } // Charting Logic var efwChart; var chartContext = document.getElementById('efwChart').getContext('2d'); // Predefined typical growth data (approximate) var typicalGrowthData = [ { ga: 20, efw: 330 }, { ga: 21, efw: 380 }, { ga: 22, efw: 440 }, { ga: 23, efw: 500 }, { ga: 24, efw: 570 }, { ga: 25, efw: 650 }, { ga: 26, efw: 730 }, { ga: 27, efw: 820 }, { ga: 28, efw: 910 }, { ga: 29, efw: 1010 }, { ga: 30, efw: 1110 }, { ga: 31, efw: 1220 }, { ga: 32, efw: 1330 }, { ga: 33, efw: 1450 }, { ga: 34, efw: 1570 }, { ga: 35, efw: 1700 }, { ga: 36, efw: 1830 }, { ga: 37, efw: 1960 }, { ga: 38, efw: 2100 }, { ga: 39, efw: 2240 }, { ga: 40, efw: 2380 }, { ga: 41, efw: 2520 }, { ga: 42, efw: 2650 } ]; var userEFWData = []; // Store user's calculated EFW points function updateChartData(currentGA, currentEFW) { // Add the latest calculated EFW to user data, removing duplicates if GA exists var existingIndex = userEFWData.findIndex(function(item) { return item.ga === currentGA; }); if (existingIndex > -1) { userEFWData[existingIndex].efw = currentEFW; } else { userEFWData.push({ ga: currentGA, efw: currentEFW }); } // Sort user data by GA to maintain order on the chart userEFWData.sort(function(a, b) { return a.ga – b.ga; }); // Limit the number of user data points shown to avoid clutter, keep last 5 calculated if (userEFWData.length > 5) { userEFWData = userEFWData.slice(-5); } updateChart(); } function updateChart() { if (efwChart) { efwChart.destroy(); } var chartLabels = []; var typicalEFWValues = []; var userEFWValues = []; // Populate labels and typical values up to a reasonable max GA (e.g., 42 weeks) for (var i = 0; i < typicalGrowthData.length; i++) { chartLabels.push(typicalGrowthData[i].ga + ' wks'); typicalEFWValues.push(typicalGrowthData[i].efw); } // Add user-calculated points to the dataset var userGALabels = []; var userEFWValuesForChart = []; for (var i = 0; i < userEFWData.length; i++) { userGALabels.push(userEFWData[i].ga + ' wks'); userEFWValuesForChart.push(userEFWData[i].efw); } efwChart = new Chart(chartContext, { type: 'line', data: { labels: chartLabels, datasets: [{ label: 'Typical EFW Range (grams)', data: typicalEFWValues, borderColor: 'rgba(0, 74, 153, 1)', backgroundColor: 'rgba(0, 74, 153, 0.2)', borderWidth: 2, fill: false, pointRadius: 0 // Hide points for the typical line }, { label: 'Your Calculated EFW (grams)', data: userEFWValuesForChart, // Use the generated labels matching user data borderColor: 'rgba(40, 167, 69, 1)', // Success color backgroundColor: 'rgba(40, 167, 69, 0.2)', borderWidth: 2, fill: false, pointRadius: 5, // Show points for user data pointHoverRadius: 7 }] }, options: { responsive: true, maintainAspectRatio: false, scales: { x: { title: { display: true, text: 'Gestational Age (Weeks)' }, ticks: { autoSkip: true, maxTicksLimit: 15 // Adjust number of x-axis labels } }, y: { title: { display: true, text: 'Estimated Fetal Weight (grams)' }, beginAtZero: true, ticks: { callback: function(value) { return value + 'g'; } } } }, plugins: { legend: { position: 'top', }, title: { display: true, text: 'Estimated Fetal Weight Progression' } } } }); } // Initialize the chart on load document.addEventListener('DOMContentLoaded', function() { updateChart(); // Initialize with empty data var faqQuestions = document.querySelectorAll('.faq-question'); faqQuestions.forEach(function(question) { question.addEventListener('click', function() { var answer = this.nextElementSibling; if (answer.style.display === 'block') { answer.style.display = 'none'; } else { answer.style.display = 'block'; } }); }); }); // Trigger calculation on input change for real-time updates var inputFields = document.querySelectorAll('.loan-calc-container input'); inputFields.forEach(function(input) { input.addEventListener('input', function() { calculateEFW(); }); }); // Initial calculation on load calculateEFW();