Fetal Weight Percentile Calculator

Q: 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.

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 2017See 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 2019Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard.; Radiology; October 1991

Last updated: Jul 02, 2024

Table of contents:

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 allowed 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:

Small for the gestational age (age of the pregnancy, counted from the conception; to calculate it, you can use gestational age calculator); and
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 the 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.) To check where the newborn's head circumference places amongst their peers, use head circumference percentile calculator.
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. To learn more about the concept, visit fundal height calculator.

How to use the fetal weight percentile calculator?

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

Enter the gender of your baby

Be specific; if you already know the baby's gender, reveal it. 🎉
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)).
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
	2.5	5	10	25	50	75	90	95	97.5
14	70	73	78	83	90	98	104	109	113
15	89	93	99	106	114	124	132	138	144
16	113	117	124	133	144	155	166	174	181
17	141	146	155	166	179	193	207	217	225
18	174	181	192	206	222	239	255	268	278
19	214	223	235	252	272	292	313	328	340
20	260	271	286	307	330	355	380	399	413
21	314	327	345	370	398	428	458	481	497
22	375	392	412	443	476	512	548	575	595
23	445	465	489	525	565	608	650	682	705
24	523	548	576	618	665	715	765	803	830
25	611	641	673	723	778	836	894	938	970
26	707	743	780	838	902	971	1,038	1,087	1,125
27	813	855	898	964	1,039	1,118	1,196	1,251	1,295
28	929	977	1,026	1,102	1,189	1,279	1,368	1,429	1,481
29	1,053	1,108	1,165	1,251	1,350	1,453	1,554	1,622	1,682
30	1,185	1,247	1,313	1,410	1,523	1,640	1,753	1,828	1,897
31	1,326	1,394	1,470	1,579	1,707	1,838	1,964	2,046	2,126
32	1,473	1,548	1,635	1,757	1,901	2,047	2,187	2,276	2,367
33	1,626	1,708	1,807	1,942	2,103	2,266	2,419	2,516	2,619
34	1,785	1,872	1,985	2,134	2,312	2,492	2,659	2,764	2,880
35	1,948	2,038	2,167	2,330	2,527	2,723	2,904	3,018	3,148
36	2,113	2,205	2,352	2,531	2,745	2,959	3,153	3,277	3,422
37	2,280	2,372	2,537	2,733	2,966	3,195	3,403	3,538	3,697
38	2,446	2,536	2,723	2,935	3,186	3,432	3,652	3,799	3,973
39	2,612	2,696	2,905	3,135	3,403	3,664	3,897	4,058	4,247
40	2,775	2,849	3,084	3,333	3,617	3,892	4,135	4,312	4,515

How to calculate the fetal weight percentile?

It's easier than you think!

1. Calculate the weight of the fetus 📉

Our fetal percentile calculator facilitates 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:

\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*}

where:

$\rm Fetal\ weight$ is given in grams (g);
$\rm AC$ means the Abdominal Circumference and is given in centimeters (cm);
$\rm FL$ means the Femur Length and is given in centimeters (cm);
$\rm HC$ means the Head Circumference and is given in centimeters (cm);
$\rm BPD$ means the Biparietal Diameter and is given in centimeters (cm); and
$\log_{10}({\rm Fetal\ weight})$ 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 at 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 ethnicities – 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.

FAQ

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:

Calculate the estimated fetal weight.
On a fetal weight percentile chart, find the intersection between the value of the estimated fetal weight and the gestational age.
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

Sex

Gestational age

Abdominal Circumference

Head Circumference

Biparietal Diameter

Femur Length

Result

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.

Baby percentileBirthweight percentileChild BMI percentile… 4 more