An estimated fetal weight calculator turns ultrasound measurements into an approximate weight for a baby before birth. A sonographer measures the head, abdomen, and thigh bone during a scan and records each value in millimeters. You enter those four numbers into the calculator above, and it applies the Hadlock formula to produce a weight in grams and in pounds and ounces. Expectant parents, midwives, and clinicians use this estimate to track growth and plan care. It is an approximation, not a precise scale reading, so always discuss the result with your provider. Many parents find the number reassuring, while others find it confusing without context. Growth scans usually happen in the third trimester, when the measurements are most meaningful.
What Estimated Fetal Weight Means
Estimated fetal weight, often shortened to EFW, is a prediction of how much a baby weighs inside the uterus. It comes from measurements taken during an ultrasound exam, usually in the third trimester. Growth scans check whether the baby is developing on a healthy curve. A weight estimate helps providers spot babies that may be smaller or larger than expected for their gestational age. Most estimates are reported in grams, the standard unit in obstetrics. Catching these patterns early allows timely monitoring and intervention when needed.
The estimate guides real decisions about care. It can influence the timing of delivery, the choice between vaginal birth and cesarean, and plans for neonatal support. It also reassures parents when growth is normal. Because no formula can weigh a baby directly, every estimate carries a margin of error. The widely cited figure is plus or minus fifteen percent, which providers keep in mind when making plans.
The Four Ultrasound Measurements
Biparietal Diameter (BPD)
The biparietal diameter is the distance between the two sides of the baby's head, measured across the widest part of the skull. It reflects head growth and correlates with gestational age. The sonographer freezes a cross-section image of the head and places calipers on the outer edges. Typical third-trimester values range from about seventy to one hundred millimeters. Head shape variations can shift this number slightly without signaling any problem.
Head Circumference (HC)
The head circumference is the distance around the baby's skull at its widest point. It grows steadily through pregnancy and is less affected by head shape than the BPD. The sonographer traces an ellipse around the skull on the same image used for the BPD. Values in the late third trimester often fall between two hundred eighty and three hundred sixty millimeters. HC pairs with the other measurements to build the full estimate. Because the skull is rigid, HC stays measurable even when the baby is curled up.
Abdominal Circumference (AC)
The abdominal circumference is measured around the baby's belly at the level of the liver and stomach. It reflects the size of the liver and the amount of stored energy in the body. The AC is the single most sensitive measurement for detecting growth problems. A small AC can signal that the baby is not getting enough nourishment. Typical late-pregnancy values range from about two hundred sixty to three hundred sixty millimeters. When the AC lags behind the head measurements, providers take a closer look at placental function.
Femur Length (FL)
The femur length is the length of the baby's thigh bone, measured from end to end on a side view. It tracks skeletal growth and is a strong indicator of overall size. The sonographer aligns the bone horizontally on screen and measures its full length. Late-pregnancy values usually sit between sixty and eighty millimeters. Shortened femur length can prompt a closer look at growth patterns and skeletal development.
The Hadlock Formula
The calculator above uses the Hadlock formula, published by Dr. Frank Hadlock and colleagues in the nineteen eighties. It remains the most widely used method for estimating fetal weight from ultrasound biometry. The version here combines all four measurements: BPD, HC, AC, and FL. Using four inputs makes it more reliable than formulas based on fewer measurements. Researchers have tested it against thousands of births across many populations.
The formula works in centimeters, so the calculator first divides each millimeter value by ten. It then computes a logarithm value: one point three two six, minus zero point zero zero three two six times AC times FL, plus zero point zero one zero seven times HC, plus zero point zero four three eight times AC, plus zero point one five eight times FL. Raising ten to that power gives the weight in grams. The result is also converted to pounds and ounces for convenience, since many families think in imperial units.
Why does the formula use logarithms? Human growth is multiplicative rather than additive, so the log scale captures how the measurements combine. The constants were fitted to thousands of ultrasound and birth records. That statistical grounding is why the method has endured for decades. Clinicians trust it because large studies back it up.
How Accurate Is the Estimate
Ultrasound weight estimates are useful but imperfect. Studies consistently show that most estimates fall within ten to fifteen percent of the actual birth weight. For a three thousand gram baby, that means the true weight could reasonably be anywhere from about two thousand five hundred fifty to three thousand four hundred fifty grams. Larger babies tend to have larger absolute errors. For this reason, providers are cautious about scheduling interventions based on a single large estimate.
Several factors affect accuracy. The baby's position can make measurements harder to take precisely. Extra amniotic fluid or maternal body tissue can blur the images. Different sonographers may place calipers slightly differently. The formula itself was built from population averages, so individual babies deviate. Treat the number as a guide for planning, not as a promise of the exact birth weight. When the estimate and the clinical picture disagree, the clinical picture usually wins. Ask your sonographer about image quality if a measurement seems surprising. A calm, well-positioned baby gives the clearest images.
How to Use the Estimated Fetal Weight Calculator
- Gather your ultrasound report. Find the biometry section of the scan report. It lists BPD, HC, AC, and FL, usually in millimeters.
- Check the units. The calculator expects millimeters. If your report uses centimeters, multiply each value by ten before entering it.
- Enter the biparietal diameter. Type the BPD value from the report into the first field. Double-check the digits against the printout.
- Enter the head circumference. Type the HC value into the second field. It should be larger than the BPD for the same scan.
- Enter the abdominal circumference. Type the AC value into the third field. This is the measurement most tied to weight.
- Enter the femur length. Type the FL value into the fourth field. All four fields must be filled for the formula to work.
- Click Calculate, then read the result. The estimate appears in grams and in pounds and ounces, with a reminder that it is only an estimate. Click Reset to clear the fields for a new set of measurements.
Worked Example 1: Thirty-Four Week Growth Scan
Inputs
At a thirty-four week growth scan, the sonographer records a BPD of eighty-five millimeters, an HC of three hundred ten millimeters, an AC of two hundred ninety millimeters, and an FL of sixty-five millimeters. The expectant mother enters eighty-five, three hundred ten, two hundred ninety, and sixty-five into the calculator above.
Calculation
The calculator converts each value to centimeters: eight point five, thirty-one point zero, twenty-nine point zero, and six point five. Plugging these into the Hadlock formula gives a log value of about three point three four. Ten raised to that power equals roughly two thousand one hundred ninety grams. Converting to imperial units, two thousand one hundred ninety grams equals about seventy-seven point three ounces. Dividing by sixteen ounces per pound gives four full pounds with thirteen point two ounces remaining.
Result
The estimated fetal weight is two thousand one hundred ninety grams, or four pounds thirteen point two ounces. This sits comfortably within the typical range for thirty-four weeks. The result is an estimate only, so the parents discuss it with their provider at the next visit.
Worked Example 2: Thirty-Eight Week Growth Scan
Inputs
At a thirty-eight week scan, the measurements are larger: BPD ninety-two millimeters, HC three hundred thirty millimeters, AC three hundred ten millimeters, and FL seventy millimeters. The parents enter ninety-two, three hundred thirty, three hundred ten, and seventy into the calculator above.
Calculation
Converted to centimeters, the inputs are nine point two, thirty-three point zero, thirty-one point zero, and seven point zero. The Hadlock formula yields a weight of about two thousand seven hundred twenty-six grams. In imperial units, that equals roughly ninety-six point one ounces. Sixteen ounces per pound gives six full pounds, leaving zero point one ounces.
Result
The estimated fetal weight is two thousand seven hundred twenty-six grams, or six pounds zero point one ounces. This is a normal range for thirty-eight weeks. With the usual fifteen percent margin, the true weight could plausibly run from about two thousand three hundred to three thousand one hundred grams.
Tips for Interpreting the Result
- Confirm the units first. Mixing millimeters and centimeters is the most common mistake. Every input must be in millimeters.
- Use measurements from the same scan. Values from different weeks cannot be combined. Growth formulas assume one point in time.
- Remember the fifteen percent margin. Mentally add and subtract fifteen percent to see the plausible range around the number.
- Watch the trend, not one number. A single estimate matters less than steady growth across scans. Ask about the growth curve.
- Do not compare with other formulas. Different equations give different answers from the same measurements. Stick with one method for tracking.
- Ask about percentiles. Providers plot the estimate against growth charts. The percentile often matters more than the raw grams.
- Bring the report to appointments. Your provider can confirm the inputs and explain what the estimate means for your care plan.
- Stay calm about small deviations. Slightly above or below average is usually normal. Providers investigate only when patterns look concerning.
- Save each scan's numbers. Keep a simple record of every growth scan. Comparing estimates over time reveals the growth pattern clearly.
Frequently Asked Questions
Understanding the Estimate
1. What is estimated fetal weight?
It is a prediction of a baby's weight before birth, calculated from ultrasound measurements. The calculator above uses the Hadlock formula with four standard measurements. The result appears in grams and in pounds and ounces. It guides care planning but is not exact.
2. How accurate is the Hadlock formula?
Most estimates fall within ten to fifteen percent of the actual birth weight. Accuracy is best for average-sized babies and near term. Position, fluid levels, and caliper placement all introduce small errors. Treat the number as a guide.
3. What does EFW mean on my ultrasound report?
EFW stands for estimated fetal weight. The report may show it in grams alongside a percentile for gestational age. The percentile compares your baby with a reference population. Ask your provider to explain your specific numbers at the visit.
4. Why does the estimate differ from the birth weight?
No formula can weigh a baby directly. Babies keep growing after the last scan, and measurements carry small errors. A scan done two weeks before delivery misses that final growth. The fifteen percent margin covers most of this gap.
5. Can the estimate predict the exact delivery date weight?
Not exactly. The formula reflects the baby's size on the day of the scan. Providers may project forward using growth rates, but that adds more uncertainty. Use the estimate for planning, not for precise predictions of birth day weight.
Measurements and the Formula
6. What are BPD, HC, AC, and FL?
They are the four standard ultrasound biometry measurements. BPD is the head width, HC the head circumference, AC the abdominal circumference, and FL the thigh bone length. Together they describe the baby's overall size. All four feed the Hadlock formula used by the calculator above.
7. Why are the inputs in millimeters?
Ultrasound machines record biometry in millimeters by convention. The calculator converts to centimeters internally because the Hadlock formula expects those units. If your report shows centimeters, multiply by ten first.
8. What if one measurement is missing?
The four-measurement Hadlock formula needs all four values. Ask the sonographer or check the full report for the missing number. Do not guess or substitute a value from another scan. An incomplete set gives no valid result, so a repeat measurement is the safe path.
9. Does a big head measurement mean a big baby?
Not necessarily. Head size varies with family traits and head shape. The formula weights all four measurements, so one large value is balanced by the rest. Your provider looks at the whole picture, including percentiles and growth trends.
10. Can I use this for twins?
Each twin needs its own set of measurements and its own calculation. Enter twin A's values, note the result, then reset and enter twin B's. Growth patterns in multiples differ from singletons, so provider guidance matters even more. Never average the two sets of measurements together.
Results and Next Steps
11. My estimate is below average. Should I worry?
Not from one number alone. Many healthy babies sit below the fiftieth percentile, which is perfectly normal. Providers watch the growth trend across scans and may order extra monitoring. Discuss the full report with your provider before drawing conclusions.
12. My estimate is above average. What happens next?
A large estimate may prompt a conversation about delivery planning. Providers consider the mother's history, pelvis, and prior births. Extra scans might track continued growth. Most large babies are born healthy without complications, which is reassuring to remember.
13. How often are growth scans done?
Most pregnancies need only the standard anatomy scan around twenty weeks. Growth scans are added when there is a medical reason, such as suspected growth restriction or excess fluid. Your provider decides the schedule based on your situation and history.
14. Does the estimate affect delivery decisions?
It can be one factor among many. Very small or very large estimates may influence timing or mode of delivery. Providers weigh the estimate with the mother's health and labor progress. No single number decides a birth plan on its own.
15. Is this calculator a substitute for medical advice?
No. It is an educational tool that applies a published formula to your numbers. Only your provider can interpret results in the context of your pregnancy. Always bring questions about the estimate to your appointments for clear answers.
CONCLUSION
An estimated fetal weight calculator turns four ultrasound measurements into a helpful approximation of a baby's weight. The Hadlock formula behind it has served clinicians for decades, and the examples above show typical results of two thousand one hundred ninety grams and two thousand seven hundred twenty-six grams. Remember the fifteen percent margin of error, watch the growth trend across scans, and always review the numbers with your provider. Used wisely, the estimate supports calm, informed decisions about the rest of your pregnancy. Share the calculator's result with your provider rather than interpreting it alone.