How to Calculate Estimated Fetal Weight by Ultrasound
Professional calculator using the Hadlock IV formula for accurate fetal weight estimation.
Enter weeks (20-42). Used for percentile calculation.
Please enter a value between 20 and 42.
Measurement across the head (mm).
Invalid BPD value.
Measurement around the head (mm).
Invalid HC value.
Measurement around the abdomen (mm).
Invalid AC value.
Length of the thigh bone (mm).
Invalid FL value.
Estimated Fetal Weight (EFW)
1950 g
Hadlock IV Formula
Weight in Lbs/Oz
4 lbs 5 oz
Est. Percentile
50th
Growth Status
Appropriate for GA
Biometric Summary
Measurement
Input (mm)
Converted (cm)
Fetal Growth Curve (Weight vs. Gestational Age)
Chart shows 10th, 50th, and 90th percentiles. Red dot indicates current calculation.
What is Estimated Fetal Weight (EFW)?
Understanding how to calculate estimated fetal weight by ultrasound is a critical component of prenatal care. Estimated Fetal Weight (EFW) is a calculation derived from biometric measurements taken during an ultrasound scan. It is not a direct measurement of weight (like a scale) but rather a mathematical prediction based on the size of fetal bones and organs.
Obstetricians and sonographers use EFW to monitor fetal growth patterns, screen for Intrauterine Growth Restriction (IUGR), or identify Macrosomia (excessive growth). While the technology is advanced, it remains an estimation with a typical margin of error of +/- 15%.
The Hadlock Formula and Mathematical Explanation
The most widely accepted method for how to calculate estimated fetal weight by ultrasound is the Hadlock formula. Specifically, the "Hadlock IV" variation is considered one of the most accurate as it utilizes four distinct biometric parameters.
The mathematical formula is logarithmic. The standard equation for Hadlock IV is:
Note: In this formula, all inputs must be in centimeters (cm), and the result is the Log10 of the weight in grams.
Variables Table
Variable
Meaning
Unit
Typical Range (3rd Trimester)
BPD
Biparietal Diameter (Head width)
cm
7.0 – 10.0 cm
HC
Head Circumference
cm
25.0 – 35.0 cm
AC
Abdominal Circumference
cm
23.0 – 36.0 cm
FL
Femur Length (Thigh bone)
cm
5.0 – 8.0 cm
Practical Examples of Fetal Weight Calculation
Example 1: 32 Weeks Gestation (Average Growth)
A patient presents for a routine growth scan at 32 weeks. The sonographer obtains the following measurements:
BPD: 81 mm (8.1 cm)
HC: 295 mm (29.5 cm)
AC: 280 mm (28.0 cm)
FL: 62 mm (6.2 cm)
Calculation: Plugging these values into the calculator yields an EFW of approximately 1,950 grams (4 lbs 5 oz). This places the fetus near the 50th percentile, indicating normal growth.
Example 2: 38 Weeks Gestation (Suspected Macrosomia)
A patient with gestational diabetes is scanned at 38 weeks to check for large size.
BPD: 94 mm (9.4 cm)
HC: 335 mm (33.5 cm)
AC: 360 mm (36.0 cm)
FL: 74 mm (7.4 cm)
Calculation: The calculator estimates the weight at 3,650 grams (8 lbs 1 oz). Given that 38 weeks is not yet full term (40 weeks), this weight is in the higher percentiles (>90th), suggesting the fetus is large for gestational age (LGA).
How to Use This EFW Calculator
Enter Gestational Age: Input the current weeks of pregnancy. This is crucial for determining the percentile.
Input Biometrics (in mm): Enter the BPD, HC, AC, and FL values from your ultrasound report. Note that most reports list these in millimeters (mm).
Review Results: The calculator instantly updates the estimated weight in grams and pounds/ounces.
Analyze the Chart: Look at the growth curve. If the red dot is between the bottom and top lines, growth is generally considered normal.
Key Factors That Affect EFW Results
When learning how to calculate estimated fetal weight by ultrasound, it is vital to understand the factors that influence accuracy:
Fetal Position: If the baby is low in the pelvis or facing the spine, obtaining accurate head or abdominal measurements becomes difficult, skewing the formula.
Amniotic Fluid Volume: Low fluid (oligohydramnios) can compress the fetus, making measurements harder, while high fluid (polyhydramnios) can distort images.
Maternal Body Habitus: High maternal BMI attenuates ultrasound waves, reducing image clarity and measurement precision.
Abdominal Shape: The AC (Abdominal Circumference) is the most weight-sensitive variable. If the fetus has a slightly compressed abdomen due to position, the weight estimate can drop significantly.
Technician Variability: Different sonographers may measure slightly differently. Inter-observer variability is a known factor in ultrasound biometry.
Biological Variation: Just like adults, fetuses have different body shapes. A long, thin baby might have the same weight as a short, stout one, but the formula might overestimate or underestimate based on bone length.
Frequently Asked Questions (FAQ)
How accurate is ultrasound weight estimation?
Ultrasound EFW typically has a margin of error of +/- 15%. For a 3,000g baby, the actual weight could be anywhere between 2,550g and 3,450g.
Why do we use bone length to calculate weight?
Bones (Femur Length, BPD) provide a stable framework for size. However, the Abdominal Circumference (AC) is the most important factor for weight, as it reflects the liver size and subcutaneous fat stores.
What is a normal percentile for fetal weight?
Generally, an EFW between the 10th and 90th percentile is considered "Appropriate for Gestational Age" (AGA). Below 10th is "Small for Gestational Age" (SGA), and above 90th is "Large for Gestational Age" (LGA).
Can I calculate EFW without all four measurements?
Yes, there are formulas that use only AC and FL (Hadlock II) or AC, FL, and BPD (Hadlock III). However, using all four (Hadlock IV) is generally preferred for maximum accuracy.
Does a high EFW mean I need a C-section?
Not necessarily. While suspected macrosomia is a factor in delivery planning, clinical decisions depend on many factors including pelvic size, diabetic status, and previous delivery history.
How often should EFW be calculated?
Growth scans are typically performed no more frequently than every 2-4 weeks. It takes time for significant growth to occur and be measurable beyond the margin of error.
Why is the result in grams?
Medical standards worldwide use the metric system (grams) for fetal weight to ensure precision. This calculator converts it to lbs/oz for convenience.
Is this calculator a diagnosis?
No. This tool is for educational and informational purposes only. Always consult your obstetrician or radiologist for medical advice and official interpretation of ultrasound results.