Estimated Fetal Weight Calculator
Based on the widely-used Hadlock formula for accurate in-utero growth assessment.
Select the unit used for your ultrasound measurements.
The diameter across the baby’s head.
The measurement around the baby’s head.
The measurement around the baby’s abdomen.
The length of the baby’s thigh bone.
Enter the gestational age in whole weeks (20-42).
What is the Formula Used to Calculate Estimated Fetal Weight?
The formula used to calculate estimated fetal weight (EFW) is a crucial tool in obstetrics for monitoring a baby’s growth and well-being during pregnancy. It uses biometric measurements taken during an ultrasound to generate an estimate of the fetus’s weight. While no formula is 100% accurate, they provide valuable data for healthcare providers to detect potential growth issues, such as fetal growth restriction (FGR) or macrosomia (a larger than average baby).
Several mathematical models exist, but the most widely adopted and trusted are the Hadlock formulas. These formulas were developed by Dr. Frank Hadlock in the 1980s and are considered a gold standard because they incorporate multiple fetal parameters, leading to a more reliable estimation. This calculator uses a common Hadlock formula that combines four key measurements for its calculation.
The Hadlock Formula for Estimated Fetal Weight
This calculator uses a well-established Hadlock formula that requires four measurements: Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). The formula is logarithmic, which helps model the non-linear pattern of fetal growth.
The specific formula is:
Log10(EFW) = 1.326 – (0.00326 * AC * FL) + (0.0107 * HC) + (0.0438 * AC) + (0.158 * FL)
After calculating the Log10(EFW), the final weight in grams is found by taking the antilog (10 to the power of the result).
| Variable | Meaning | Unit | Typical Range (32 weeks) |
|---|---|---|---|
| EFW | Estimated Fetal Weight | Grams (g) | 1500 – 2200 g |
| BPD | Biparietal Diameter | Centimeters (cm) | 7.5 – 8.9 cm |
| HC | Head Circumference | Centimeters (cm) | 28.0 – 31.5 cm |
| AC | Abdominal Circumference | Centimeters (cm) | 25.0 – 30.0 cm |
| FL | Femur Length | Centimeters (cm) | 5.6 – 6.6 cm |
Practical Examples
Example 1: Mid-Third Trimester Scan
A patient has an ultrasound at 33 weeks gestation.
- Inputs: HC: 30.5 cm, AC: 29.0 cm, FL: 6.4 cm
- Calculation: Applying the Hadlock formula gives an estimated weight.
- Result: The estimated fetal weight is approximately 2095 grams, or 4 lbs 10 oz. This is a healthy weight for this gestational age. Explore our gestational age calculator for more insights.
Example 2: Approaching Full Term
An ultrasound is performed at 38 weeks gestation to check the baby’s position and size.
- Inputs: HC: 33.8 cm, AC: 34.5 cm, FL: 7.2 cm
- Calculation: The inputs are used in the formula to compute the EFW.
- Result: The estimated fetal weight is approximately 3350 grams, or 7 lbs 6 oz. This helps the care team plan for delivery. Our pregnancy due date calculator can help you track your timeline.
How to Use This Estimated Fetal Weight Calculator
- Select Units: First, choose whether your ultrasound report provides measurements in centimeters (cm) or millimeters (mm). The calculator will convert them automatically.
- Enter Measurements: Carefully input the four required biometric parameters from your ultrasound report: Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).
- Enter Gestational Age: Input the gestational age in weeks at the time of the scan. This is crucial for the percentile chart.
- Calculate: Click the “Calculate” button. The results will update automatically as you type.
- Interpret the Results: The calculator provides the EFW in grams and pounds/ounces. The chart shows how this weight compares to the 10th, 50th, and 90th percentiles for the specified gestational age, giving you a clear picture of your baby’s growth.
Key Factors That Affect Fetal Weight
Many factors can influence a baby’s weight. The formula used to calculate estimated fetal weight provides a snapshot, but these underlying factors contribute to the final number:
- Genetics: The height and weight of the parents play a significant role in determining the baby’s size.
- Maternal Health: Conditions like gestational diabetes can lead to larger babies (macrosomia), while high blood pressure or preeclampsia can sometimes result in lower birth weights.
- Maternal Nutrition and Weight Gain: A healthy, balanced diet supports appropriate fetal growth. Both insufficient and excessive weight gain can impact the baby’s final weight.
- Placental Function: The placenta delivers oxygen and nutrients to the baby. Any issues with placental health can restrict growth. For more details, see our fetal percentile calculator.
- Number of Fetuses: Twins, triplets, and other multiples often have lower individual birth weights than singletons.
- Fetal Sex: On average, male babies tend to be slightly heavier than female babies at birth.
Frequently Asked Questions (FAQ)
- How accurate is the estimated fetal weight calculator?
- Ultrasound-based weight estimation has a margin of error, typically around +/- 15% in the third trimester. It is a screening tool, not a diagnostic one, but it is highly effective for tracking growth trends over time.
- Why are four measurements used?
- Combining measurements of the head (BPD, HC), abdomen (AC), and a long bone (FL) provides a more holistic and accurate picture of the baby’s overall size than using just one or two parameters. The abdominal circumference is particularly sensitive to changes in fetal nutrition.
- What if one measurement is unavailable?
- While the four-parameter formula is most accurate, other Hadlock formulas exist that use two or three parameters. However, for the best estimation, all four are recommended. This calculator requires all four for its primary calculation.
- What do the percentiles on the chart mean?
- Percentiles compare your baby’s weight to a large population of babies at the same gestational age. The 50th percentile is the average. A weight in the 10th percentile means the baby is smaller than 90% of babies, while a weight in the 90th means the baby is larger than 90% of babies.
- Can I use measurements in millimeters (mm)?
- Yes. Simply select “Millimeters (mm)” from the unit dropdown. The calculator will automatically convert the values to centimeters (cm) for the formula, as that is the standard unit for Hadlock calculations.
- What is considered a normal fetal weight?
- A “normal” weight generally falls between the 10th and 90th percentiles. Weights outside this range are not necessarily a problem but may prompt your doctor to perform further monitoring. To understand more about these measurements, see our guide on what is BPD in pregnancy.
- Does gestational age affect the calculation?
- The gestational age does not change the weight calculation itself, but it is essential for interpreting the result. The percentile chart relies entirely on the gestational age to provide a meaningful comparison.
- When is this calculation typically performed?
- EFW is most commonly calculated during third-trimester ultrasounds (from 28 weeks onwards) to monitor growth, especially if there are concerns about the baby being too small or too large.
Related Tools and Internal Resources
Continue exploring important pregnancy and health topics with our other specialized calculators:
- Pregnancy Due Date Calculator: Estimate your due date based on your last menstrual period or conception date.
- Ovulation Calculator: Pinpoint your most fertile days to help you conceive.
- BMI Calculator: Check your Body Mass Index to ensure a healthy weight before and during pregnancy.
- Gestational Age Calculator: Calculate how far along you are in your pregnancy.
- Fetal Growth Percentile Calculator: See how your baby’s measurements compare to standard growth charts.
- What is BPD in Pregnancy?: An in-depth article explaining this key ultrasound measurement.