Ultrasound scans provide standard anatomical assessments of the developing fetus. Measuring skeletal bone lengths, cranial diameters, and abdominal structures enables clinicians to evaluate gestational age, estimate fetal weight, and check placental blood flow trends. Use our free Pregnancy Sonography Calculator to analyze your ultrasound measurements.
How to Use the Pregnancy Sonography Calculator
- Select your target tab: Toggle between Biometry (BPD, HC, AC, FL, CRL), Weight, Doppler Waveforms, or Growth parameters depending on what is on your scan.
- Enter biometric measurements: Provide measurements in millimeters (mm) from your ultrasound report.
- Review fetal outputs: Click Calculate to render composite gestational age, estimated fetal weight (EFW), growth percentile curves, and blood flow status.
Formula & Calculation Method
Individual biometric values are converted to estimated gestational ages (GA) in weeks using validated regression equations:
BPD GA = BPD² * 0.000326 + BPD * 0.5596 + 8.541 HC GA = HC² * 0.000011 + HC * 0.1677 + 8.5151 AC GA = AC² * 0.000008 + AC * 0.1463 + 4.3594 FL GA = FL² * 0.0053 + FL * 0.7585 + 11.618
Fetal weight is estimated using Hadlock equations that combine AC, FL, and BPD/HC to calculate EFW in grams.
Example Calculation
If your ultrasound report lists a BPD of 70 mm, HC of 265 mm, AC of 230 mm, and FL of 52 mm:
- Calculated BPD GA: ~28 weeks 1 day.
- Calculated HC GA: ~28 weeks 3 days.
- Calculated AC GA: ~27 weeks 6 days.
- Composite Gestational Age: Average is 28 weeks 1 day.
- Estimated Fetal Weight: Hadlock equation yields ~1,200 grams.
Reference Data: Ultrasound Fetal Biometry Standards
| Parameter | Full Name | Clinical Description | Accuracy Range |
|---|---|---|---|
| CRL | Crown-Rump Length | Measurement from top of head (crown) to bottom of buttocks (rump). | ±3–5 days (1st trimester dating standard) |
| BPD | Biparietal Diameter | Transverse width of the fetal skull from temple to temple. | ±1–2 weeks (2nd trimester dating baseline) |
| HC | Head Circumference | Outer circumference of the fetal head. | ±1.5–2 weeks (assesses cranial size trends) |
| AC | Abdominal Circumference | Outer circumference of the fetal abdomen. | ±2–3 weeks (primary parameter for estimating weight) |
| FL | Femur Length | Length of the fetal thigh bone (longest bone in the body). | ±2 weeks (tracks long-bone skeletal growth) |
What the Result Means
Ultrasound percentiles indicate how baby dimensions compare to healthy reference populations. Values between the 10th and 90th percentile show typical growth, while values below 10th percentile may require closer monitoring to check for intrauterine growth restriction (IUGR).
Common Mistakes to Avoid
- Confusing millimeters with centimeters: Ultrasound dimensions are standardly reported in millimeters. Using centimeters in this calculator will lead to erroneous results.
- Expecting exact weights: Estimated fetal weight is only an estimate. Hadlock equations have an expected variance of up to 15%.
- Overlooking Doppler indicators: Doppler study ratios (like Umbilical PI and MCA PI) require specialist interpretation. Elevated indexes do not guarantee complications but signal closer review.
When This Calculator Is Useful
Ultrasound Report Decoding
Decode complex acronyms (BPD, FL, HC, AC) and find your gestational age estimates in seconds.
EFW Percentile Verification
Calculate your estimated fetal weight percentile to check if growth is on track compared to peers.
Doppler CPR Estimations
Obtain calculated Cerebroplacental Ratios (CPR) to help review placental blood flow status.
Gestation Date Alignment
Compare your ultrasound composite age with your period-based due dates to identify discrepancies.
This tool is for educational purposes only. Always consult your obstetrician, sonographer, or midwife to interpret medical diagnostic reports.