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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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).
Last reviewed: January 2026
Worked Examples
Example 1: Normal Growth at 34 Weeks
Example 2: Small for Gestational Age at 36 Weeks
Background & Theory
The Estimated Fetal Weight Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the Estimated Fetal Weight Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
Sources & References
- 1Hadlock FP et al. Estimation of Fetal Weight with Use of Head, Body, and Femur Measurements. Radiology 1985
- 2Shepard MJ et al. An Evaluation of Two Equations for Predicting Fetal Weight by Ultrasound. Am J Obstet Gynecol 1982
- 3ACOG Practice Bulletin No. 204: Fetal Growth Restriction. Obstet Gynecol 2019
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.
References
- Hadlock FP et al. Estimation of Fetal Weight with Use of Head, Body, and Femur Measurements. Radiology 1985
- Shepard MJ et al. An Evaluation of Two Equations for Predicting Fetal Weight by Ultrasound. Am J Obstet Gynecol 1982
- ACOG Practice Bulletin No. 204: Fetal Growth Restriction. Obstet Gynecol 2019
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy