Fetal Weight Percentile Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Steven Wooding
Based on research by
Kiserud T, Piaggio G, Carroli G, Widmer M, Carvalho J, Neerup Jensen L, et al. The World Health Organization Fetal Growth Charts: A Multinational Longitudinal Study of Ultrasound Biometric Measurements and Estimated Fetal Weight. PLoS Medicine (January 2017)See 2 more sources
Blue NR, Savabi M, Beddow ME, Katukuri VR, Fritts CM, Izquierdo LA, Chao CR. The Hadlock Method Is Superior to Newer Methods for the Prediction of the Birth Weight Percentile. Journal of Ultrasound in Medicine (March 2019)Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology (October 1991)
Last updated: May 09, 2022

Our fetal weight percentile calculator computes your child's growth and compares it to the general population. Our BPD, HC, AC, and FL calculator uses all the necessary ultrasound fetal measurements to compute your child's weight at a given week of the pregnancy.

We use the Hadlock equation and WHO multinational growth charts. 👶👶🏾👶🏽👶🏻

Want to know more? Follow the article below to discover fetal weight percentile chart by week, and details of our baby weight predictor calculator. We'll also cover the baby percentiles during pregnancy as well as the SGA and LGA definitions.

What percentile is my baby, and what does it mean?

We use percentiles to measure values that are extremely variable in the population - especially in children. For example, we can't give a precise value of height that a healthy 2-year old boy should achieve - it all depends on his individual and genetic characteristics. 🔬

However, doctors love all the hard data and numbers that help them make critical clinical decisions. They needed a tool that allows quick and precise evaluation of their patients.

So how did we tackle this problem? 🤔

Percentiles are a great solution! Instead of giving us a precise number, they inform us about the range in which your baby's height or weight should fall. Thanks to percentiles, we can tell how your child's health looks when compared to the general population and detect any significant abnormalities.

When we talk about fetal weight abnormalities, two most important definitions come to mind:

  1. Small for the gestational age (age of the pregnancy, counted from the conception)
  2. Large for the gestational age

💡 SGA - Small for Gestational Age babies are all babies that fall below the 10th percentile of the population, when it comes to their EFW percentile. It means that over 90% of the population grew larger than they did.

💡 LGA - Large for Gestational Age babies exceed the 90th percentile of their Estimated Fetal Weight. It means that only 10% of population is larger than them.

These two explanations are based on an Estimated Fetal Weight (EFW) calculations that use the ultrasound technique to evaluate a baby's weight inside its mother's uterus.

To estimate the fetal weight, we use the following measurements:

  • Abdominal circumference - circumference of the baby's belly, measured at the liver's height. (Advanced: an umbilical portion of the left portal vein should be in the center of the abdomen.)

  • Head circumference - measured at the point where specific brain structures are visible. (Advanced: plane that traverses the thalami and cavum septum pellucidum.)

  • Biparietal diameter - length of a distance between two parietal bones of a baby's skull.

  • Femur length - a length of the baby's thigh bone.

Our fetal growth calculator uses all four of the enumerated measurements; additionally, we can also measure the humerus length.

💡 We could also measure the estimated fetal weight by assessing the fundal height - this method, however, is not as accurate.

You've learned everything about fetal weight percentiles - go on and check our other percentile-themed tools:

How to use the fetal weight percentile calculator?

Our estimated fetal weight percentile calculator needs just a few easy steps:

  1. Enter the gender of your baby

    Be specific; if you already know the baby's gender, reveal it. 🎉

  2. Enter the gestational age

    This calculator is for fetuses older than 14 weeks. (We use a different type of measurement in younger pregnancies: the Crown-rump length (CRL)).

  3. Find all the enumerated ultrasound measurements

    Remember to choose the correct unit - imperial or metric.

🔅 Our tool also works as a large and small for gestational age calculator: along with the results, you will receive feedback concerning your child's weight's location on the chart and its meaning. Read more about LGA/SGA in the section above.

Fetal growth chart

Here you can find the fetal weight week by week for the fetuses of an unknown gender ♀️/♂️:

Gestational Age (weeks)Estimated Fetal Weight (g) by percentile

Along with monitoring the baby's growth and weight, we can also check if the mother's pregnancy weight gain is also within a healthy range. 🤰

How to calculate the fetal weight percentile?

It's easier than you think!

1. Calculate the weight of the fetus 📉

Our fetal percentile calculator faciliates the process of computing the fetus weight, based on the ultrasound measurements; however, if you'd like to calculate it yourself, you're more than welcome to try the equation we used:

log10(Fetal weight)=1.3596(0.00386ACFL)+(0.0064HC)+(0.00061BPDAC)+(0.0424AC)+(0.174FL)\footnotesize \begin{align*} &\log{}_{10}\left(\text{Fetal weight}\right) = \\ &1.3596 - (0.00386 \cdot \text{AC} \cdot \text{FL}) +\\ &(0.0064 \cdot \text{HC}) + (0.00061 \cdot \text{BPD} \cdot \text{AC}) \\ &+ (0.0424 \cdot \text{AC}) + (0.174 \cdot \text{FL}) \end{align*}


  • Fetal weight is given in grams (g);
  • AC means the Abdominal Circumference and is given in centimeters (cm);
  • FL means the Femur Length and is given in centimeters (cm);
  • HC means the Head Circumference and is given in centimeters (cm);
  • BPD means the Biparietal Diameter and is given in centimeters (cm); and
  • log10(Fetal weight) means the fetal weight logarithm of the base of 10.

2. Find the percentile 📈

When you already know the fetus's weight, all you need to do is check the fetal growth charts or tables for a given gestational age.

  • Find your baby's gestational age on the bottom of the page: draw a vertical line starting from that point.
  • Find its weight on the left or right edge of the chart, and draw a horizontal line beginning from that point.
  • Mark the crossing of these two lines with a dot.
  • Check which fetal weight percentile line on the chart is located the closest to the dot you've just drawn.

❗ Our birth weight calculator for the US is a tool prepared for measuring children of different ethinicites - despite that fact, you shouldn't use it as a single source of clinical knowledge. Always consult your doctor, and double check any measurements that may have any impact on clinical practice.


What is a percentile?

A percentile is a score used in statistics and medicine to indicate where a certain measurement places among the same measurement in its peers.

When we say that a measurement belongs to the k-th percentile, we mean that k % of the measurements in a population are below the one we are considering. Percentiles are largely used when assessing widely variable quantities, like the heights and weights of individuals.

What is the estimated fetal weight?

The estimated fetal weight is the baby's weight at a specific time of the pregnancy. Because the measurement cannot be taken directly, it is estimated using an ultrasound scan, which finds:

  • Abdominal and head circumference (AC and HC);
  • Biparietal diameter (the distance between parietal bones in the skull) (BPD); and
  • Femur length (occasionally also humerus length) (HL).

Plug those measurements in the following formula to find the estimated fetal weight:
log₁₀(Fetal weight) = 1.3596 - (0.00386 × AC × FL) + (0.0064 × HC) + (0.00061 × BPD × AC) + (0.0424 × AC) + (0.174 × FL).

How do I find the fetal weight percentile?

To find the fetal weight percentile, follow these steps:

  1. Calculate the estimated fetal weight.
  2. On a fetal weight percentile chart, find the intersection between the value of the estimated fetal weight and the gestational age.
  3. The intersection will fall close to a line on the graph: each line is associated with a specific percentile, which in this case, corresponds to your baby's percentile.

Which percentile is a fetus with estimated fetal weight 250 g at 18 weeks?

75ᵗʰ percentile. To calculate the percentile, find a fetal weight percentile chart and find the intersection between the lines corresponding to the values:

  • Estimated fetal weight 250 g; and
  • Gestational age 18 weeks.

The closest line to the point is the 75ᵗʰ percentile, which means that the baby is heavier than 75% of all other babies the same age.

Łucja Zaborowska, MD, PhD candidate
Gestational age
Abdominal Circumference
Head Circumference
Biparietal Diameter
Femur Length
Estimated fetal weight
Your baby is in the 50th percentile! 👶
✅ Your baby's weight is within normal range.
    • Y axis: fetal weight in grams (g).
    • X axis: gestational age in weeks.
Check out 7 similar percentile calculators 👶
Baby percentileBirthweight percentileChild BMI percentile… 4 more
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