Calculate Weight of Baby in Womb
Use this professional calculator to estimate fetal weight using standard ultrasound measurements (BPD, AC, FL, HC). Based on the clinically validated Hadlock IV formula.
Typically between 60mm (13 weeks) and 360mm (40 weeks).
Please enter a valid AC between 50 and 400 mm.
Length of the thigh bone. Typically 10mm to 80mm.
Please enter a valid FL between 10 and 90 mm.
Diameter of the head across the ears. Typically 20mm to 105mm.
Please enter a valid BPD between 20 and 110 mm.
Circumference of the baby's head. Typically 70mm to 350mm.
Please enter a valid HC between 60 and 400 mm.
Estimated Fetal Weight (EFW)
1,500 g
Weight in Pounds/Ounces
3 lbs 5 oz
Weight in Kilograms
1.50 kg
Hadlock Log Value
3.176
Formula Used: Hadlock IV (Log10) using AC, FL, BPD, and HC. Measurements converted to cm for calculation.
| Metric | Value | Clinical Unit |
|---|---|---|
| Combined Biometrics | AC:260, FL:56, BPD:78, HC:280 | mm |
| Calculated Grams | 1500 | g |
| Variance (+/- 15%) | 1275 – 1725 | g |
Chart compares your calculated Estimated Fetal Weight (EFW) against standard 50th percentile weights for different gestational weeks.
Table of Contents
What is Calculate Weight of Baby in Womb?
The ability to calculate weight of baby in womb (Estimated Fetal Weight or EFW) is a critical component of prenatal care. It uses biometric measurements taken during an ultrasound to estimate how much the fetus weighs at a specific point in gestation. This calculation helps obstetricians and midwives monitor growth patterns, identify potential restrictions (Intrauterine Growth Restriction – IUGR), or anticipate macrosomia (large baby).
While a scale cannot be used in utero, mathematical regression models developed by researchers like Hadlock, Shepard, and Campbell allow us to calculate weight of baby in womb with a reasonable degree of accuracy—typically within 10-15% of the actual birth weight. This tool is primarily designed for expecting parents who have their ultrasound reports and wish to understand the metrics, as well as sonography students verifying calculations.
Fetal Weight Formulas and Mathematical Explanation
To accurately calculate weight of baby in womb, medical professionals rely on regression formulas. The most widely accepted method worldwide is the Hadlock IV Formula. This formula utilizes four key biometrics to generate a logarithmic estimate of weight.
The standard Hadlock IV equation is:
Log10(EFW) = 1.3596 – 0.00386(AC × FL) + 0.0064(HC) + 0.00061(BPD × AC) + 0.0424(AC) + 0.174(FL)
Note: In the formula above, all input measurements (AC, FL, HC, BPD) must be in centimeters (cm). The result is the Log base 10 of the weight in grams. The final weight is obtained by calculating 10 to the power of the result.
Variables Definition Table
| Variable | Meaning | Measurement Unit | Typical Range (30 Weeks) |
|---|---|---|---|
| AC | Abdominal Circumference | cm (input as mm) | 24.0 – 28.0 cm |
| FL | Femur Length | cm (input as mm) | 5.4 – 6.0 cm |
| BPD | Biparietal Diameter | cm (input as mm) | 7.5 – 8.2 cm |
| HC | Head Circumference | cm (input as mm) | 27.0 – 29.0 cm |
Practical Examples (Real-World Use Cases)
Example 1: The 32-Week Checkup
Sarah is at her 32-week scan. Her technician provides the following measurements in millimeters. She wants to calculate weight of baby in womb to see if the baby is growing on track.
- AC: 280 mm (28.0 cm)
- FL: 62 mm (6.2 cm)
- BPD: 82 mm (8.2 cm)
- HC: 300 mm (30.0 cm)
Calculation: Using the calculator, these inputs yield an estimated weight of approximately 1,950 grams (about 4 lbs 5 oz). This aligns well with the 50th percentile for 32 weeks, reassuring Sarah that growth is normal.
Example 2: Monitoring for Macrosomia
Jessica is 38 weeks pregnant and has gestational diabetes. Her doctor wants to monitor the baby's size.
- AC: 360 mm (36.0 cm) – Larger than average
- FL: 74 mm (7.4 cm)
- BPD: 94 mm (9.4 cm)
- HC: 340 mm (34.0 cm)
Result: The calculator estimates the weight at roughly 3,650 grams (8 lbs 1 oz). Since there are still 2 weeks to go, this data helps the medical team discuss delivery options, as the baby is trending large.
How to Use This Fetal Weight Calculator
- Gather Your Report: Locate the biometry section on your latest ultrasound report. Look for AC, FL, BPD, and HC.
- Check Units: Ensure your numbers are in millimeters (mm). If your report uses cm, multiply by 10 (e.g., 26.5 cm = 265 mm).
- Input Values: Enter the values into the respective fields:
- Abdominal Circumference (AC)
- Femur Length (FL)
- Biparietal Diameter (BPD)
- Head Circumference (HC)
- Review Results: The tool will instantly calculate weight of baby in womb. Review the grams, kilograms, and pounds/ounces display.
- Analyze the Variance: Remember that ultrasound estimates have a margin of error (+/- 15%). The "Variance" row in the table shows this range.
Key Factors That Affect Fetal Weight Results
When you calculate weight of baby in womb, several biological and technical factors influence the final number.
- Fetal Position: If the baby is low in the pelvis or in a difficult position, obtaining an accurate Head Circumference (HC) or BPD can be challenging, skewing the weight calculation.
- Amniotic Fluid Volume: Low fluid (oligohydramnios) can make structures harder to visualize, while high fluid (polyhydramnios) allows for clearer images but may be associated with different growth patterns.
- Maternal Tissue: High maternal BMI can attenuate ultrasound waves, reducing image clarity and measurement precision.
- Genetic Potential: Parents' height and weight play a significant role. A fetus destined to be constitutionally small may measure small but be perfectly healthy.
- Placental Health: The placenta supplies nutrients. Placental insufficiency can lead to lower AC measurements relative to the head size (asymmetric growth restriction).
- Operator Experience: The skill of the sonographer in obtaining standard cross-sections is crucial for accurate biometry.
Frequently Asked Questions (FAQ)
How accurate is an ultrasound weight estimate?
Ultrasound weight estimates are generally accurate within +/- 15%. This means if the calculator predicts 2,000g, the actual weight could realistically be between 1,700g and 2,300g. Accuracy decreases as the pregnancy progresses towards full term.
Why is Abdominal Circumference (AC) so important?
AC is the most variable parameter and reflects the liver size and subcutaneous fat. It is the best single predictor of fetal nutrition and growth restriction.
Can I calculate weight of baby in womb without all 4 measurements?
Yes, there are formulas that use only AC and FL, or AC, FL, and BPD. However, using all four (AC, FL, BPD, HC) generally provides the most robust estimate by accounting for head shape variations.
What is a normal weight for 30 weeks?
At 30 weeks, the average fetal weight is approximately 1,300 to 1,500 grams (about 3 lbs). However, a wide range is considered normal.
Does this calculator diagnose growth problems?
No. This tool provides an estimate based on math. Diagnosis of IUGR or macrosomia requires a doctor's assessment of growth velocity over time, Doppler studies, and amniotic fluid levels.
Why is my baby measuring different weeks for different parts?
It is common for the head, abdomen, and legs to grow at slightly different rates. As long as the overall trend is consistent, minor discrepancies are usually normal.
Is a heavy baby a problem?
Not necessarily. While fetal macrosomia (weight > 4,000g or 4,500g) carries slightly higher risks during delivery, many large babies are born without complications.
How often should I check the weight?
Ultrasounds for growth are typically performed no more frequently than every 2-3 weeks to allow enough time for measurable change to occur.
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