Printed on 3/17/2026
For informational purposes only. This is not medical advice.
The Fetal Weight Calculator uses the Hadlock formula (1985) to estimate fetal weight (EFW) from four standard ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). This is the most widely used and validated formula for estimated fetal weight worldwide. The result is classified as small for gestational age (SGA, <10th percentile), appropriate for gestational age (AGA, 10th–90th), or large for gestational age (LGA, >90th). EFW has a margin of error of ±15% and is crucial for detecting growth restriction (IUGR) and macrosomia. Use alongside [Gestational Age Calculator](/tools/gestational-age) to confirm gestational age before interpreting percentile classification.
Formula: Hadlock: log₁₀(EFW) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD)(AC) − 0.00386(AC)(FL). All measurements in cm.
Input four standard second- or third-trimester ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). These are routinely measured in every fetal anatomy scan and growth scan, typically recorded in millimeters.
Provide the current gestational age in weeks (confirmed by first-trimester ultrasound or last menstrual period). Gestational age is essential for percentile classification — a 1500g fetus at 30 weeks is SGA, but the same weight at 26 weeks is normal. Confirm gestational age with the Gestational Age Calculator if needed.
The calculator applies the Hadlock formula to produce an estimated fetal weight in grams and pounds, then classifies it as SGA (below 10th percentile), AGA (10th–90th percentile), or LGA (above 90th percentile) using standard Hadlock growth charts. Serial measurements plotted over time provide the most clinically meaningful assessment.
Obstetricians, maternal-fetal medicine
Identify fetuses below the 10th percentile for gestational age. Serial EFW measurements every 2–4 weeks establish growth velocity, which is more meaningful than any single value. Suspected IUGR prompts additional Doppler studies, amniotic fluid assessment, and closer monitoring.
OB/GYN physicians, midwives
Estimate fetal size in pregnancies complicated by gestational diabetes or maternal obesity. EFW above the 90th percentile or above 4000g prompts delivery planning discussions, as macrosomia increases risk of shoulder dystocia, birth trauma, and operative delivery.
High-risk obstetrics teams
Guide timing and mode of delivery based on estimated fetal size. ACOG guidelines suggest considering cesarean delivery for EFW above 4500g in diabetic mothers or 5000g in non-diabetic mothers to reduce birth injury risk.
Sonographers, radiologists
Calculate EFW as part of the standard 18–22 week anatomy scan to establish a baseline growth trajectory. Normal EFW at this stage provides reassurance; any concern triggers follow-up growth ultrasound at 28–32 weeks.
High-risk obstetrics, perinatology
Monitor fetal growth in pregnancies complicated by hypertension or preeclampsia, where placental insufficiency can impair fetal growth. Serial EFW every 2–3 weeks helps determine optimal delivery timing by balancing fetal maturity against ongoing placental dysfunction.
Expectant parents
Help parents understand fetal size relative to gestational age norms. Expressing EFW in both grams and pounds/ounces, alongside a percentile context, helps make ultrasound results more meaningful and actionable during prenatal appointments.
The Hadlock 4-parameter formula (BPD + HC + AC + FL) is more accurate than 2- or 3-parameter versions. If any single measurement is suboptimal due to fetal position or oligohydramnios, the estimate may be unreliable — note this in the clinical context rather than substituting a different formula.
Among the four biometric parameters, abdominal circumference (AC) has the strongest correlation with fetal weight. AC is directly affected by glycogen stores in the fetal liver, which is why diabetic fetuses show macrosomia most prominently in the abdomen. If only one measurement is suboptimal, a poor AC reading has the greatest impact on EFW accuracy.
A single EFW near the 10th percentile is much less informative than two measurements 3–4 weeks apart showing crossing of percentile lines. Fetuses with normal growth velocity but constitutionally small size (genetic potential) are distinguished from true IUGR by their growth trajectory, not a single data point.
The Hadlock formula's ±15% margin of error is most problematic when it matters most — in very small and very large fetuses. A 3500g EFW could be anywhere from 2975g to 4025g. For borderline cases near the 10th or 90th percentile, clinical judgment, Doppler findings, and amniotic fluid assessment should weigh equally with EFW.
EFW percentile classification is entirely dependent on accurate gestational age. If gestational age is uncertain by more than 1–2 weeks, percentile classification becomes unreliable. Always use the earliest ultrasound date for gestational age, ideally from a first-trimester crown-rump length measurement.
Your estimated fetal weight (EFW) is classified by gestational age percentile. A weight below the 10th percentile is classified as small for gestational age (SGA), which may indicate intrauterine growth restriction (IUGR) and warrants further evaluation with serial ultrasounds, amniotic fluid assessment, and umbilical artery Doppler studies. A weight between the 10th and 90th percentile is appropriate for gestational age (AGA). A weight above the 90th percentile is large for gestational age (LGA), which is associated with gestational diabetes, maternal obesity, and increased risk of birth complications including shoulder dystocia.
It is important to understand that the Hadlock formula has an inherent margin of error of approximately plus or minus 15%. This means a fetus estimated at 3000 grams could actually weigh anywhere from 2550 to 3450 grams. A single measurement near a percentile boundary should be interpreted cautiously, and serial growth assessments are more reliable than any single estimate.
Use this calculator when fetal biometry measurements (BPD, HC, AC, FL) are available from a second- or third-trimester ultrasound to estimate fetal weight. It is particularly important when screening for growth abnormalities — suspected IUGR in pregnancies complicated by hypertension, preeclampsia, or placental insufficiency, or suspected macrosomia in pregnancies complicated by gestational diabetes.
Serial EFW assessments every 2-4 weeks are used to establish a growth velocity trend, which is more clinically meaningful than a single measurement. The calculator is most accurate between 20 and 36 weeks of gestation; accuracy decreases at the extremes of fetal size and in the late third trimester.
The Hadlock formula, while the most widely validated, was developed from a relatively homogeneous population and may not be equally accurate across all ethnic groups. It tends to underestimate weight in very large fetuses and overestimate weight in very small fetuses, which is problematic precisely in the clinical scenarios where accuracy matters most.
Ultrasound-based EFW is operator-dependent, and measurement variability between sonographers can be significant. The formula requires all four biometric parameters (BPD, HC, AC, FL) for optimal accuracy — if any measurement is suboptimal due to fetal position, oligohydramnios, or maternal body habitus, the estimate may be less reliable. This calculator does not replace clinical assessment, and delivery planning should never be based solely on a single EFW measurement.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
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