Fetal Weight Estimate Calculator
Estimate fetal weight from three routine ultrasound measurements — head circumference, abdominal circumference and femur length — using the Hadlock formula. Enter values in mm or cm and get the result in grams, kilograms and pounds.
Estimate Fetal Weight from Ultrasound Measurements
Fetal Weight Estimate Formula
Where:
- \(EFW\) — estimated fetal weight, in grams (g)
- \(HC\) — head circumference, in centimeters (cm)
- \(AC\) — abdominal circumference, in centimeters (cm)
- \(FL\) — femur length, in centimeters (cm)
This is the Hadlock equation based on three parameters (HC, AC and FL). It expresses the estimated weight as a power of 10, which means the weight itself is found by calculating \( EFW = 10^{\log_{10}(EFW)} \). All measurements must be in centimeters before the formula is applied.
Fetal Weight Estimate – Calculation Example
Example: a third-trimester ultrasound reports a head circumference of 320 mm, an abdominal circumference of 300 mm and a femur length of 65 mm. These are the calculator's default values, so you can reproduce the result directly.
Convert the measurements to centimeters: \( HC = 32 \text{ cm}, \; AC = 30 \text{ cm}, \; FL = 6.5 \text{ cm} \).
\[ \log_{10}(EFW) = 1.326 - 0.00326 \times 30 \times 6.5 + 0.0107 \times 32 + 0.0438 \times 30 + 0.158 \times 6.5 = 3.3737 \]
\[ EFW = 10^{3.3737} \approx 2365 \text{ g} \]
Result: the estimated fetal weight is approximately 2365 g, which is about 2.37 kg or 5.21 lb.
How to Use the Calculator
- Enter the Head circumference (HC) from your ultrasound report and select whether the value is in mm or cm.
- Enter the Abdominal circumference (AC) and choose its unit (mm or cm).
- Enter the Femur length (FL) and choose its unit (mm or cm).
- Read the Estimated fetal weight in grams, together with the same result converted to kilograms and pounds. The panel also shows the measurements used and a note on the limitations of ultrasound estimates.
Ultrasound reports usually state measurements in millimeters, which is why mm is the default unit. If your report gives values in centimeters, simply switch the unit selector — no manual conversion is needed.
What Is Estimated Fetal Weight?
Estimated fetal weight (EFW) is the predicted weight of a baby before birth, calculated from measurements taken during an ultrasound. It is one of the most closely watched numbers in the third trimester because it shows whether the baby is growing along a healthy curve for its gestational age. An estimate that is too small may indicate fetal growth restriction, while an estimate that is too large may suggest macrosomia, which can affect plans for delivery.
The calculator uses the Hadlock formula, published in 1985 and still one of the most widely used equations worldwide. Hadlock and colleagues derived it from ultrasound data of large groups of fetuses, using head circumference, abdominal circumference and femur length. There are several Hadlock variants (two, three, four or five parameters); this one uses three parameters, which keeps the input simple while remaining accurate enough for everyday use.
Because the formula is based on population averages, the result is a statistical estimate, not a measurement. If a baby is actually 2500 g, the estimate might reasonably fall anywhere from roughly 2250 g to 2750 g. Clinicians therefore interpret the EFW together with gestational age, growth trends across repeated scans, amniotic fluid levels and the mother's clinical picture — never in isolation. You can pair this calculator with our gestational age calculator to see where the weight falls for the baby's age, and with the amniotic fluid index calculator to check another key growth indicator.
Interpreting the Result
A single EFW number means little on its own; what matters most is the percentile for gestational age. As a general reference, below the 10th percentile raises concern about fetal growth restriction, and above the 90th percentile suggests the baby may be larger than average. Only your maternity care team can place the estimate on the correct growth chart and decide whether follow-up is needed.
Trends are more informative than single values. If growth was normal at 28 weeks and has slowed by 36 weeks, that pattern matters more than one absolute number, even if both readings are technically within normal limits. This is also why estimating fetal weight earlier in pregnancy is less useful — small measurement errors translate into proportionally larger weight errors as the pregnancy advances.
Assumptions and Limitations
- Measurement accuracy dominates the result. Because the formula combines several measurements, small errors in each one compound. Studies show ultrasound EFW is typically within about 10–15% of the actual birth weight, but individual cases can deviate more.
- The formula assumes a typical fetal body shape. It was derived from population data and does not account for individual differences in body composition, which is a known source of error in babies of diabetic mothers and babies at the extremes of growth.
- Later estimates are less precise. In the last weeks of pregnancy the baby fills the pelvis, deeper parts of the abdomen become harder to image, and error margins grow.
- Enter measurements, not estimates. Use the raw HC, AC and FL values from your report. Do not enter an estimated weight, percentile or gestational age into the measurement fields.
- Sanity checks are built in. The calculator rejects values above 50 cm for head and abdominal circumference and above 12 cm for femur length, which helps catch unit mix-ups (for example, entering millimeters while cm is selected).
Practical Tips
If your report lists both millimeter and centimeter values for the same measurement, they should agree (e.g. 320 mm = 32.0 cm); if they do not, one of them is likely a typo, so double-check before calculating. Keep in mind that different hospitals may use different Hadlock variants or other formulas such as Shepard or Warsof, so your report's stated EFW may differ slightly from this calculator's result even with identical measurements.
To track growth over time, record each scan's EFW along with the date, and compare it to the crown rump length measurements from early pregnancy and the milestones in your due date timeline. If you have already given birth, you may find the newborn weight loss calculator useful for monitoring your baby's first days.
Finally, remember that this tool is for information only. It does not replace a medical review of your ultrasound images, and you should always discuss the EFW and its implications with your obstetrician or midwife.
Frequently Asked Questions
How accurate is the estimated fetal weight compared to the actual birth weight?
Ultrasound EFW is typically within about 10–15% of the true birth weight. For a 3000 g estimate, that means the baby could reasonably weigh anywhere from roughly 2550 g to 3450 g. Accuracy is best in the middle range of weights and drops for very small or very large babies, and in the last weeks before delivery. The further the scan is from birth, the more time the baby has to grow away from the estimate.
What is a normal fetal weight at 32, 36 and 40 weeks?
As rough averages: about 1700–1800 g at 32 weeks, 2600–2900 g at 36 weeks, and 3300–3500 g at 40 weeks. But 'normal' spans a wide range — a healthy 36-week baby might weigh anywhere from about 2200 g to 3500 g. What matters is the percentile for gestational age, not the absolute number. Below the 10th or above the 90th percentile usually triggers closer monitoring by your care team.
Why does my result differ from the EFW on my ultrasound report?
Hospitals use different formulas — there are several Hadlock variants (with two, four or five parameters) plus alternatives like Shepard, Warsof and INTERGROWTH-21st. Even with identical measurements, these formulas give slightly different weights, often varying by 5–10%. Measurement technique also differs between scans and sonographers. A small discrepancy is expected; only worry about large differences, and discuss those with your provider.
Can I use biparietal diameter (BPD) instead of head circumference?
No — this calculator needs the head circumference, because the Hadlock three-parameter formula uses HC, AC and FL. BPD is a linear measurement of the head's width, while HC is the circumference around it. Some reports list both. If yours only gives BPD, ask your provider for the HC value, or use a calculator built on a BPD-based formula such as Shepard or Warsof.
How do I convert grams to pounds and ounces?
Divide grams by 453.59237 to get pounds. For example, 2900 g ≈ 6.39 lb, which is 6 lb and about 6 oz (multiply the decimal part, 0.39, by 16). Conversely, multiply pounds by 453.59237 to get grams. The calculator shows grams, kilograms and pounds automatically, so you only need to convert by hand if you want the ounces part.
Can the estimate predict my baby's exact birth weight or whether I'll need a C-section?
No. EFW is a statistical estimate, not a prediction, and the gap grows with every week between the scan and delivery. A large estimate may prompt a discussion about delivery options, but decisions about vaginal birth or C-section also consider the mother's pelvis, previous births, glucose levels and labor progress. Always discuss delivery planning with your obstetrician rather than relying on a calculator.
Is it safe to estimate fetal weight early in pregnancy, before 24 weeks?
You can enter the measurements, but the result is not clinically meaningful that early. The Hadlock formula was designed for the second half of pregnancy, when HC, AC and FL are large enough to measure reliably. Before about 20–24 weeks, tiny measurement errors produce large percentage errors in the weight, and growth is usually tracked by crown-rump length or biometry charts instead. Use this calculator mainly for third-trimester scans.