fetal weight calculator

What is a fetal weight calculator?

A fetal weight calculator estimates how much a baby may weigh in the womb based on ultrasound measurements. Clinicians usually call this value Estimated Fetal Weight (EFW). Since the baby cannot be weighed directly before birth, formulas are used to convert body measurements into a weight estimate.

These estimates are useful for routine growth tracking and may help identify patterns such as growth restriction or larger-than-expected size. However, every estimate has a margin of error, especially later in pregnancy.

Measurements used in the calculator

BPD (Biparietal Diameter)

BPD is the distance across the baby’s head from one parietal bone to the other. It is often used to track head growth and can contribute to EFW formulas.

HC (Head Circumference)

HC is the circumference around the baby’s head. It can improve estimate accuracy by adding broader head growth information.

AC (Abdominal Circumference)

AC is one of the most influential measurements in fetal weight estimation because abdominal size tends to reflect nutrition and growth status.

FL (Femur Length)

FL measures the length of the thigh bone. Combined with head and abdomen measurements, it helps produce a balanced estimate of body size.

How the formulas work

This page includes common obstetric equations:

• Hadlock (HC + AC + FL): widely used in clinical settings.
• Hadlock (BPD + HC + AC + FL): uses four biometric values for a broader estimate.
• Shepard (BPD + AC): an older, still-referenced method in some contexts.

The formulas calculate a logarithmic value first, then convert that value into grams. Results are shown in grams, kilograms, and pounds/ounces for easier interpretation.

How to interpret your result

• Single estimate: One value alone is less informative than a trend over multiple scans.
• Growth trend: Providers compare measurements over time to assess whether growth remains steady.
• Gestational context: If you enter gestational age, this tool gives a rough percentile estimate based on a reference median curve.

A percentile is not a diagnosis by itself. Many healthy babies are naturally small or large.

Accuracy and limitations

EFW is an estimate, not an exact birth weight. Differences can occur due to fetal position, scan quality, technician technique, maternal factors, and biological variation. In many cases, real birth weight can differ by around 10% (sometimes more) from ultrasound predictions.

Because of this uncertainty, EFW should always be interpreted together with full prenatal information: fluid levels, Doppler findings, maternal health, prior growth scans, and clinical judgment.

Best practices when using this calculator

• Use the most recent ultrasound values and keep units in millimeters.
• Recheck entries for typing mistakes before calculating.
• Use one formula consistently when tracking trends.
• Discuss concerns with your prenatal team rather than relying on one online result.

Frequently asked questions

Can this calculator predict exact birth weight?

No. It provides an estimate from biometric inputs. Actual birth weight may be higher or lower.

Why do different formulas give different values?

Each equation was developed from different patient samples and variables, so results naturally vary.

Should I worry if percentile is low or high?

A single percentile does not automatically mean a problem. Your obstetric clinician looks at trend, anatomy, blood flow, and maternal factors before drawing conclusions.

Medical disclaimer

This calculator is for education and informational use only. It does not replace professional prenatal care, diagnostic ultrasound interpretation, or medical advice. If you have concerns about fetal growth, contact your obstetrician, midwife, or maternal-fetal medicine specialist.