Fetal Growth Calculator
Fetal Growth Analysis
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About Your Result
This percentile shows how your baby's measurement compares to other babies at the same gestational age.
This calculator is an educational tool, not medical advice. Fetal growth assessment is complex. Please consult your healthcare provider for an accurate interpretation of your ultrasound.
What is a Fetal Growth Calculator?
A Fetal Growth Calculator is a tool used to estimate your baby's size and growth by calculating their percentile. During pregnancy, especially in the second and third trimesters, doctors perform ultrasounds to take biometric measurements of the fetus. This calculator compares your baby's measurements to a large, standardized dataset of healthy fetuses at the same gestational age.
The result is given as a percentile, which is a key indicator of fetal well-being. A percentile of 50 means your baby's measurement is exactly average. A percentile of 20 means your baby's measurement is larger than 20% of babies (and smaller than 80%) at the same age. Doctors use these percentiles to ensure the baby is growing at a steady and appropriate rate.
How Fetal Growth is Measured (Ultrasound Biometry)
Your doctor or sonographer will measure several key parts of the fetus to assess growth. This calculator uses the four most common measurements, which are part of the Hadlock formula, a standard method for estimating fetal weight and growth.
BPD (Biparietal Diameter)
This is the measurement of the diameter across your baby's head, from one parietal bone to the other (ear to ear).
HC (Head Circumference)
This is the circumference (the distance around) your baby's head. It is often used with BPD to ensure the head shape is normal.
AC (Abdominal Circumference)
This is the measurement around your baby's abdomen. The AC is a critical indicator of fetal size and is especially important for monitoring growth in the third trimester as it reflects fetal fat stores and liver size.
FL (Femur Length)
This is the length of the femur, which is the long bone in your baby's thigh. It is the longest bone in the body and provides a good indication of the baby's overall length and growth.
How the Percentile Calculation Works
This calculator uses a standard statistical method to find the percentile. It is based on finding the Z-score, which measures how many standard deviations your baby's measurement is from the average (mean) for their exact gestational age.
Calculation Formula:
1. Find the Mean (μ) and Standard Deviation (σ)
The calculator has a built-in data table (from Hadlock, et al.) that stores the average (mean) measurement and standard deviation for each biometric (BPD, HC, AC, FL) for each week of gestation.
2. Calculate the Z-Score
Z-Score = (Your Baby's Measurement - Mean for Age) / Standard Deviation for Age
3. Convert Z-Score to Percentile
The Z-score is converted to a percentile. A Z-score of 0 is the 50th percentile. A negative Z-score is below average, and a positive Z-score is above average.
Example Calculation
- Gestational Age: 30 weeks + 0 days
- Measurement Type: AC (Abdominal Circumference)
- Your Baby's Measurement: 265 mm
Calculation:
1. Find Data for 30w 0d (AC):
- Standard Mean (Average): 258.8 mm
- Standard Deviation: 12.6 mm
3. Convert to Percentile: A Z-score of +0.49 corresponds to the 69th percentile.
Result: This means the baby's abdominal circumference is larger than 69% of babies at 30 weeks gestation.
Fetal Growth FAQs
A "normal" or "Average for Gestational Age" (AGA) percentile is generally considered to be anywhere between the 10th and 90th percentiles. The vast majority of healthy babies fall within this wide range. A baby at the 15th percentile and a baby at the 85th percentile are both considered normal, just on different ends of the growth spectrum.
A fetus is diagnosed as "Small for Gestational Age" (SGA) if their estimated weight or a key measurement (like the Abdominal Circumference) falls below the 10th percentile. This means the baby is smaller than 90% of other babies at the same gestational age. This can be perfectly fine, or it could be a sign of a growth issue (Fetal Growth Restriction), which would require closer monitoring by your doctor.
A fetus is diagnosed as "Large for Gestational Age" (LGA) if their estimated weight or a key measurement falls above the 90th percentile. This means the baby is larger than 90% of other babies at the same age. This is often related to genetics (tall parents), but it can also be a sign of conditions like gestational diabetes, which would require follow-up testing and monitoring.
This calculator uses published, standard datasets (like Hadlock) just as an ultrasound machine does. However, the final percentile is highly dependent on the skill of the sonographer. A measurement that is off by even 1-2 millimeters can shift the percentile significantly. Ultrasound measurements are best thought of as an *estimate* (with a +/- 15-20% margin of error for weight), not a precise number. The *trend* of growth over multiple ultrasounds is often more important than a single reading.