Fetal Weight Estimator
Estimate fetal weight using standard obstetric measurements
How to Use This Tool
Start by entering the gestational age in weeks, between 20 and 42 weeks. Select whether your measurement inputs are in centimeters or millimeters. Enter the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) from your prenatal ultrasound report. Click Calculate Weight to generate results, or Reset to clear all fields. Use the Copy Results button to save the output to your clipboard.
All measurements should be taken from a recent obstetric ultrasound performed by a licensed provider. Double-check values against your report to ensure accuracy.
Formula and Logic
This tool uses the Hadlock et al. 1985 formula, the most widely accepted clinical method for estimating fetal weight (EFW) using four standard ultrasound measurements:
- Biparietal Diameter (BPD): Distance between the two sides of the fetal head.
- Head Circumference (HC): Circumference of the fetal head.
- Abdominal Circumference (AC): Circumference of the fetal abdomen.
- Femur Length (FL): Length of the fetal thigh bone.
The formula calculates the base-10 logarithm of estimated weight, which is then converted to grams. Weight percentile is calculated relative to the 50th percentile (median) weight for the input gestational age, using standard American College of Obstetricians and Gynecologists (ACOG) fetal growth benchmarks.
Practical Notes
Estimated fetal weight is an approximation, not a definitive measurement. Individual variation in fetal growth is normal, and results may differ from actual birth weight by 10-15% on average.
- Always consult your obstetrician or prenatal care provider to interpret results. This tool is not a substitute for professional medical advice.
- Ultrasound measurements may have minor variability between providers or machines. Use measurements from a single recent ultrasound for best accuracy.
- Fetal weight percentiles indicate relative growth compared to other fetuses at the same gestational age, not absolute health status.
- Estimates are only valid for gestational ages between 20 and 42 weeks, as standard growth benchmarks do not apply outside this range.
Why This Tool Is Useful
Expectant parents can use this tool to understand fetal growth trends between prenatal visits. Obstetricians and prenatal care providers can use it to quickly verify weight estimates from ultrasound reports. It provides a clear breakdown of weight in multiple units and percentile ranking, which helps track growth patterns over time.
The tool eliminates manual calculation errors and provides instant, easy-to-read results with visual percentile indicators, making it accessible for both clinical and personal use.
Frequently Asked Questions
How accurate is this fetal weight estimate?
Estimates using the Hadlock formula are typically within 10-15% of actual birth weight. Accuracy decreases for fetuses with growth restrictions or macrosomia (excessive growth), and results should always be confirmed by a medical professional.
Can I use this tool before 20 weeks of pregnancy?
No, standard fetal weight estimation formulas and growth percentiles are only validated for gestational ages 20 weeks and above. Earlier ultrasounds focus on structural development rather than weight estimation.
What does a low weight percentile mean?
A percentile below 10th may indicate fetal growth restriction, while a percentile above 90th may indicate macrosomia. However, percentile alone does not diagnose health conditions, and your provider will consider other factors like maternal health and blood flow studies.
Additional Guidance
For the most accurate results, use measurements from a level II obstetric ultrasound, which provides more detailed and precise measurements than standard early ultrasounds. If your ultrasound report includes estimated fetal weight, compare it to the results from this tool to verify consistency.
Track weight estimates over multiple ultrasounds to identify growth trends, rather than relying on a single measurement. Sudden drops or increases in percentile should be discussed with your prenatal care provider immediately.
Remember that fetal weight is only one indicator of health. Your provider will assess overall fetal well-being using multiple metrics, including heart rate, movement, and amniotic fluid levels.