How to Calculate the Fetal Weight
Professional Estimated Fetal Weight (EFW) Calculator & Guide
Weight Percentiles for 32 Weeks
| Percentile | Weight (grams) | Weight (lbs/oz) |
|---|
Fetal Growth Curve
What is How to Calculate the Fetal Weight?
Understanding how to calculate the fetal weight is a critical component of prenatal care. Estimated Fetal Weight (EFW) is a calculation derived from ultrasound measurements that helps obstetricians and midwives assess whether a baby is growing at a healthy rate inside the womb. Unlike weighing a baby on a scale after birth, fetal weight cannot be measured directly; it must be mathematically estimated using biometric data.
This calculation is primarily used to screen for fetal growth restriction (FGR), where a baby is smaller than expected, or macrosomia, where a baby is significantly larger than average. Knowing how to calculate the fetal weight allows healthcare providers to make informed decisions regarding delivery timing and method.
Common misconceptions include the belief that belly size correlates perfectly with fetal weight. In reality, maternal factors like fluid volume and abdominal muscle tone can obscure the true size of the fetus, making ultrasound calculations the gold standard for estimation.
Fetal Weight Formula and Mathematical Explanation
The most widely accepted method for how to calculate the fetal weight is the Hadlock formula. While there are several variations (Hadlock I, II, III, IV), the four-parameter formula (Hadlock IV) is considered one of the most accurate as it accounts for head shape, abdominal girth, and bone length.
The Hadlock IV Formula
The mathematical equation uses a logarithmic scale to predict weight based on four inputs converted to centimeters:
Log10(Weight) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD × AC) – 0.00386(AC × FL)
After calculating the log value, the result is raised to the power of 10 to get the weight in grams.
Variables Table
| Variable | Meaning | Unit | Typical Range (3rd Trimester) |
|---|---|---|---|
| BPD | Biparietal Diameter (Head Width) | cm | 7.0 – 10.0 cm |
| HC | Head Circumference | cm | 25.0 – 35.0 cm |
| AC | Abdominal Circumference | cm | 23.0 – 36.0 cm |
| FL | Femur Length (Thigh Bone) | cm | 5.0 – 8.0 cm |
Practical Examples (Real-World Use Cases)
Example 1: The 32-Week Checkup
Consider a fetus at 32 weeks gestation. The sonographer measures the following:
- BPD: 81 mm (8.1 cm)
- HC: 295 mm (29.5 cm)
- AC: 280 mm (28.0 cm)
- FL: 62 mm (6.2 cm)
Calculation: Using the calculator above, these inputs yield an estimated weight of approximately 1,950 grams (4 lbs 5 oz). This places the fetus near the 50th percentile, indicating perfectly average growth.
Example 2: Screening for Macrosomia
At 38 weeks, a patient with gestational diabetes undergoes a scan. The measurements are:
- BPD: 96 mm
- HC: 340 mm
- AC: 360 mm (Larger than average)
- FL: 74 mm
Result: The calculator estimates the weight at roughly 3,600 grams (7 lbs 15 oz). While this is a healthy weight, the large Abdominal Circumference (AC) relative to the head suggests the baby is absorbing excess sugar, a key marker for macrosomia managed by diet or insulin.
How to Use This Fetal Weight Calculator
Learning how to calculate the fetal weight with this tool is straightforward. Follow these steps for the most accurate estimation:
- Enter Gestational Age: Input the current week of pregnancy (between 20 and 42). This determines the percentile ranking.
- Input Biometrics: Enter the BPD, HC, AC, and FL values from your ultrasound report. Ensure you enter them in millimeters (mm), which is the standard unit on most ultrasound machines.
- Review Results: The calculator instantly updates the Estimated Fetal Weight in grams and pounds.
- Analyze the Chart: Look at the "Fetal Growth Curve" to see where your result sits compared to the average line. If the point is significantly above or below the line, consult your healthcare provider.
Key Factors That Affect Fetal Weight Results
When learning how to calculate the fetal weight, it is essential to understand the biological and environmental factors that influence the final number.
1. Genetic Potential
Just as parents vary in height and weight, so do fetuses. Tall, heavier parents are statistically more likely to have larger babies. This is not a pathology but a genetic variance.
2. Placental Health
The placenta is the "bank" from which the fetus draws resources. If the placenta is not functioning optimally (placental insufficiency), the fetus may not receive adequate nutrients, leading to a lower calculated weight (IUGR).
3. Maternal Nutrition and Health
Maternal diet plays a huge role. Severe caloric restriction can reduce fetal weight, while excessive intake, particularly of simple sugars, can increase it. Conditions like anemia can also restrict growth.
4. Gestational Diabetes
Uncontrolled blood sugar in the mother crosses the placenta, triggering the fetus to produce more insulin. Insulin acts as a growth hormone, often leading to excessive growth, particularly in the abdominal area (high AC).
5. Fetal Gender
On average, male fetuses tend to be slightly heavier than female fetuses at the same gestational age. Most general calculators use a unisex average, but slight variations exist.
6. Measurement Error
The accuracy of how to calculate the fetal weight depends heavily on the skill of the sonographer. A slight angle change in measuring the abdomen (AC) can alter the weight estimate by 10-15%. This is why EFW is considered an estimate with a margin of error of +/- 15%.
Frequently Asked Questions (FAQ)
Ultrasound weight estimates generally have a margin of error of about 10% to 15%. This means a baby estimated to be 3,000g could actually weigh anywhere between 2,550g and 3,450g.
The AC is the most variable parameter and the most sensitive indicator of fetal nutrition. It reflects the size of the liver and subcutaneous fat, which are the first to change if the baby is getting too much or too little energy.
Clinical palpation (feeling the belly) and fundal height measurements can give a rough idea, but they are far less accurate than learning how to calculate the fetal weight using ultrasound biometrics.
A weight below the 10th percentile for the gestational age is typically classified as "Small for Gestational Age" (SGA) or Fetal Growth Restriction (FGR), requiring closer monitoring.
A weight above the 90th percentile is considered "Large for Gestational Age" (LGA). If the weight exceeds 4,000g or 4,500g at term, it is termed macrosomia.
While the formula can be applied to twins, twin growth patterns often slow down in the third trimester compared to singletons. Specific twin growth charts should be consulted.
Ultrasound scans for growth are typically spaced at least 2 to 3 weeks apart. Measuring too frequently can lead to errors because the interval is too short to detect significant growth trends.
Not necessarily. While high fetal weight is a factor, doctors also consider pelvic size, the baby's position, and maternal history before recommending a Cesarean section.
Related Tools and Internal Resources
- Pregnancy Due Date Calculator – Estimate your delivery date based on LMP.
- Pregnancy BMI Calculator – Monitor healthy maternal weight gain.
- Understanding Ultrasound Measurements – A deep dive into BPD, HC, AC, and FL.
- Gestational Diabetes Diet Guide – Manage blood sugar to control fetal weight.
- Risks of Macrosomia – Understanding the implications of a large baby.
- Fetal Growth Percentile Charts – Downloadable charts for tracking progress.