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.