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Calculate fetal weight percentile by week using estimated fetal weight, gestational age, and growth charts with our easy online calculator.
A Fetal Weight Percentile calculator helps estimate how a baby's weight compares with other babies at the same stage of pregnancy. It uses the estimated fetal weight (EFW), gestational age, and a fetal growth reference.
Our online fetal weight percentile calculator by week makes this process simple. You can enter ultrasound measurements and get an estimated fetal weight. The result can then be compared with a suitable fetal weight percentile chart.
This tool can help parents, students, and health professionals understand fetal growth. It can also show whether the estimated weight is below, near, or above the expected range for that week, which may indicate fetal growth restriction.
Fetal weight percentile shows the baby's estimated weight compared with babies at the same gestational age.
For example, a fetal weight at the 50th percentile means the estimated weight is near the middle of the reference group. A weight at the 10th percentile is lower than most babies of the same gestational age.
The percentile is not the baby's exact weight. Ultrasound fetal weight is an estimate and can have measurement error. The growth chart or standard used also affects the result.
The calculator first estimates fetal weight from ultrasound measurements. One commonly used Hadlock equation is:
Fetal weight = 10^X
X = 1.326 − 0.00326 × (AC × FL) + 0.0107 × HC + 0.0438 × AC + 0.158 × FL
Here, HC means head circumference, AC means abdominal circumference, and FL means femur length. These measurements are entered in centimeters.
After finding estimated fetal weight, the calculator can find the percentile from a reference mean and standard deviation.
Z-score = (EFW − Reference Mean) / Reference SD
Fetal weight percentile = Normal CDF(Z-score) × 100
The normal CDF converts the Z-score into a percentile. A correct estimated fetal weight percentile calculator must use reference data that match the gestational age and selected growth standard.
Suppose an ultrasound gives these values:
HC = 29.0 cm
AC = 27.0 cm
FL = 6.2 cm
Gestational age = 32 weeks
Using the Hadlock equation:
X = 1.326 − 0.00326 × (27 × 6.2) + 0.0107 × 29 + 0.0438 × 27 + 0.158 × 6.2
X ≈ 3.2528
Now calculate estimated fetal weight:
EFW = 10^3.2528
EFW ≈ 1,790 grams
So the estimated fetal weight is about 1.79 kg.
Next, suppose the correct reference data for that gestational age have a mean of 1,950 grams and an SD of 250 grams.
Z-score = (1,790 − 1,950) / 250
Z-score = −0.64
A Z-score of about −0.64 corresponds to about the 26th percentile.
So the estimated fetal weight is approximately at the 26th percentile for that reference group.
This example shows why gestational age matters. A fetal weight that is normal at one week may be low or high at another week.
A fetal weight percentile chart compares estimated fetal weight with gestational age. Common percentile lines include the 3rd, 10th, 50th, 90th, and 97th percentiles.
The 50th percentile represents the middle of the reference group. The 10th percentile means the estimated weight is lower than about 90% of the reference group.
Different growth standards can give different percentile results. For this reason, a fetal weight by week percentile result should always state which growth standard was used.
A fetal weight percentile alone doesn't diagnose a growth problem. Doctors may also consider growth over time, ultrasound measurements, blood flow, maternal factors, and other clinical findings.
A fetal weight percentile calculator is used during pregnancy. It estimates fetal weight from ultrasound measurements.
A baby weight percentile calculator is used after birth. It normally compares an infant's measured weight with babies of the same age and sex.
A birth weight percentile calculator is different again. It compares a newborn's actual birth weight with babies of the same gestational age and sex.
So, fetal weight, newborn weight, and infant weight should not be treated as the same measurement.
The Fetal Weight Percentile calculator offers a quick way to understand estimated fetal growth. It combines ultrasound measurements, gestational age, an EFW formula, and a suitable fetal growth reference.
For the best result, always use the growth standard that matches the clinical report. Remember that an ultrasound weight is an estimate, not an exact measurement.
A fetal weight percentile shows how an estimated baby's weight compares with babies at the same gestational age.
The 50th percentile is close to the middle of the reference group. It doesn't mean the baby must weigh exactly the same as the average baby.
During pregnancy, enter the ultrasound measurements and gestational age into a fetal weight percentile calculator. The tool estimates fetal weight and compares it with the appropriate growth reference.
It can provide a useful estimate, but ultrasound EFW has measurement error. The result also depends on the ultrasound measurements and growth standard used.
It means the estimated fetal weight is below the 10th percentile for the selected reference group. Clinical assessment is needed to determine what this means for an individual pregnancy.
Yes. A fetal weight by week percentile chart can help compare estimated weight across gestational ages. The chart should match the growth standard used by the calculator.
No. Fetal weight percentile uses an estimated weight during pregnancy. Birth weight percentile uses the newborn's actual birth weight after delivery.
No. Infant growth should use an age- and sex-appropriate infant growth chart. A fetal growth calculator is designed for pregnancy.