Estimated Fetal Weight Calculator

Estimate fetal weight from ultrasound measurements using Hadlock and Shepard formulas. Enter values for instant results with step-by-step formulas.

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Formula

Hadlock: log10(EFW) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD)(AC) - 0.00386(AC)(FL)

Where BPD = Biparietal Diameter (cm), HC = Head Circumference (cm), AC = Abdominal Circumference (cm), FL = Femur Length (cm). Measurements are taken in millimeters and converted to centimeters for the formula. The Shepard formula uses only BPD and AC: log10(EFW) = -1.7492 + 0.166(BPD) + 0.046(AC) - 0.002646(AC)(BPD).

Worked Examples

Example 1: Normal Growth at 34 Weeks

Problem: 34-week fetus with BPD 85mm, HC 310mm, AC 300mm, FL 65mm.

Solution: Using Hadlock 4-parameter formula:\nBPD=8.5cm, HC=31.0cm, AC=30.0cm, FL=6.5cm\nlog10(EFW) = 1.3596 + 0.0064(31.0) + 0.0424(30.0) + 0.174(6.5) + 0.00061(8.5)(30.0) - 0.00386(30.0)(6.5)\nlog10(EFW) = 1.3596 + 0.1984 + 1.272 + 1.131 + 0.15555 - 0.7527 = 3.3639\nEFW = 10^3.3639 = 2313 grams

Result: Hadlock EFW: ~2313g (5.10 lbs) | Percentile: 25th-75th (AGA) | Normal growth

Example 2: Small for Gestational Age at 36 Weeks

Problem: 36-week fetus with BPD 82mm, HC 300mm, AC 270mm, FL 63mm.

Solution: Using Hadlock 4-parameter formula:\nBPD=8.2cm, HC=30.0cm, AC=27.0cm, FL=6.3cm\nlog10(EFW) = 1.3596 + 0.0064(30.0) + 0.0424(27.0) + 0.174(6.3) + 0.00061(8.2)(27.0) - 0.00386(27.0)(6.3)\nlog10(EFW) = 1.3596 + 0.192 + 1.1448 + 1.0962 + 0.13493 - 0.65637 = 3.271\nEFW = 10^3.271 = 1868 grams

Result: Hadlock EFW: ~1868g (4.12 lbs) | Percentile: Below 10th (SGA) | Further evaluation recommended

Frequently Asked Questions

What is estimated fetal weight and why is it important?

Estimated fetal weight (EFW) is a calculated approximation of a fetus's weight in utero, derived from ultrasound measurements of fetal biometry. It is one of the most clinically significant calculations in obstetric care because abnormal fetal growth, whether too small or too large, is associated with increased risk of adverse pregnancy outcomes. Small for gestational age (SGA) fetuses below the 10th percentile may indicate placental insufficiency, genetic conditions, or intrauterine growth restriction, and carry higher risks of stillbirth. Large for gestational age (LGA) fetuses above the 90th percentile are associated with gestational diabetes, shoulder dystocia, and the need for cesarean delivery.

What ultrasound measurements are needed for fetal weight estimation?

Four standard ultrasound measurements are used for comprehensive fetal weight estimation. The biparietal diameter (BPD) measures the widest diameter of the fetal skull from one parietal bone to the other. The head circumference (HC) is the circumference of the fetal head at the level of the thalami and cavum septi pellucidi. The abdominal circumference (AC) is measured at the level of the fetal stomach and umbilical vein junction with the portal sinus. The femur length (FL) measures the longest diaphysis of the femur bone. Of these, the abdominal circumference is the single most important measurement for weight estimation, as it reflects soft tissue mass including the liver and subcutaneous fat.

How often should fetal weight be estimated during pregnancy?

In uncomplicated pregnancies, fetal biometry is typically assessed at the anatomy scan around 18 to 22 weeks and may be repeated in the third trimester around 32 to 36 weeks if clinically indicated. For high-risk pregnancies involving conditions such as hypertension, diabetes, prior growth-restricted fetus, or multiple gestation, growth scans are performed more frequently, typically every 2 to 4 weeks. Serial assessments should be spaced at least 2 weeks apart because the inherent measurement error of ultrasound can obscure real growth changes over shorter intervals. The growth velocity between serial scans is often more clinically meaningful than a single weight estimate, as it reveals whether the fetus is maintaining, accelerating, or decelerating its growth trajectory.

What factors can affect the accuracy of fetal weight estimation?

Multiple factors influence the accuracy of ultrasound-based fetal weight estimation. Operator technique and experience significantly affect measurement quality, with inter-observer variability of up to 10 to 15 percent for abdominal circumference measurements. Maternal body habitus, particularly obesity with a BMI above 35, reduces image quality and measurement accuracy. Fetal position, oligohydramnios (low amniotic fluid), and polyhydramnios (excess fluid) can all make measurements more difficult. Gestational age extremes reduce accuracy, with estimates being less reliable before 24 weeks and after 40 weeks. Fetal anomalies such as hydrocephalus or skeletal dysplasias invalidate standard formulas designed for normally proportioned fetuses.

Can fetal weight estimation predict the mode of delivery?

While estimated fetal weight is one factor in delivery planning, it alone is not sufficient to predict or determine the mode of delivery. The American College of Obstetricians and Gynecologists (ACOG) does not recommend routine cesarean delivery based solely on suspected macrosomia unless the estimated weight exceeds 5000 grams in non-diabetic mothers or 4500 grams in diabetic mothers. Studies have consistently shown that ultrasound-based weight estimates have limited predictive accuracy for macrosomia, with positive predictive values of only 40 to 60 percent. Clinical decisions about delivery mode should incorporate multiple factors including maternal pelvis dimensions, fetal presentation, labor progress, prior delivery history, and the presence of comorbid conditions.

What is the difference between EFW and actual birth weight?

Estimated fetal weight from ultrasound differs from actual birth weight by approximately 10 to 15 percent on average, with 95 percent of estimates falling within plus or minus 15 percent of true weight when using the Hadlock formula. Several factors contribute to this discrepancy. Ultrasound measures dimensions and uses mathematical models to estimate weight, but cannot directly measure mass. The formulas assume standard body proportions and composition, which vary among individual fetuses. The time interval between the last ultrasound and delivery also affects accuracy, as the fetus continues to grow approximately 200 to 250 grams per week in the third trimester. Additionally, measurement technique variability introduces random error. For clinical purposes, the estimate should be considered a range rather than a precise number.