What is Fetal Weight Percentile?
Fetal weight percentile is a statistical measure that compares your baby's estimated weight to other babies at the same gestational age. It tells you what percentage of babies weigh less than yours at a given point in pregnancy. For example, if your baby is at the 75th percentile, it means that 75% of babies at the same gestational age weigh less, and 25% weigh more.
Healthcare providers use fetal weight percentiles as one of the key indicators of fetal health and development. A baby's percentile helps doctors assess whether growth is progressing normally throughout the pregnancy. It is important to understand that percentiles are not grades — being at the 30th percentile does not mean your baby is unhealthy. What matters most is that the baby's growth follows a consistent trajectory over time, rather than suddenly dropping or spiking in percentile rank.
Fetal weight percentiles are established using large population studies that track thousands of pregnancies. The most commonly referenced growth charts come from the World Health Organization (WHO), the INTERGROWTH-21st project, and national standards such as those from Hadlock. These charts define what a "normal" range of weights looks like at each week of gestation, typically considering the 10th through 90th percentiles as the normal range.
Tracking fetal weight percentile across multiple ultrasound visits is especially valuable. A single measurement provides a snapshot, but the trend over time reveals whether the baby is growing at an appropriate rate. A baby consistently at the 20th percentile throughout pregnancy is typically healthy — the baby may simply be constitutionally small. However, a baby that drops from the 60th percentile to the 15th percentile over a few weeks may warrant further investigation, as this could indicate growth restriction or other complications.
What is Estimated Fetal Weight?
Estimated fetal weight (EFW) is a calculation derived from ultrasound measurements of the baby's body. Since it is not possible to directly weigh a baby in the womb, sonographers measure specific anatomical landmarks and plug those values into mathematical formulas to estimate the weight. The most commonly measured parameters are:
- Abdominal Circumference (AC): The measurement around the baby's abdomen at the level of the liver and stomach. This is the single most important measurement for weight estimation because the abdomen reflects soft tissue mass, subcutaneous fat, and liver size — all of which correlate strongly with overall weight.
- Head Circumference (HC): The measurement around the widest part of the baby's skull. Head size correlates with brain development and overall body size, contributing to the weight estimate.
- Biparietal Diameter (BPD): The distance between the two parietal bones on opposite sides of the skull, measured from the outer edge of one to the inner edge of the other. This provides a cross-sectional measurement of the head.
- Femur Length (FL): The length of the thigh bone, which is the longest bone in the fetal skeleton. Femur length correlates with overall skeletal size and provides information about linear growth.
It is important to note that estimated fetal weight has an inherent margin of error. Even the most accurate formulas typically have an error range of 10–15%, meaning the actual birth weight can differ from the ultrasound estimate by this amount. The accuracy tends to decrease at the extremes — very small or very large babies are harder to estimate precisely. Despite this limitation, EFW remains one of the most valuable tools in prenatal care for monitoring fetal growth and identifying potential complications early.
The Hadlock Method
The Hadlock method, developed by Dr. Frank P. Hadlock and colleagues in the 1980s, is one of the most widely validated and commonly used formulas for estimating fetal weight. Hadlock published several variations of the formula using different combinations of ultrasound measurements, but the four-parameter formula (using AC, HC, BPD, and FL) is considered the most accurate because it incorporates the most anatomical information.
The formula used in this calculator is:
Where AC, HC, BPD, and FL are all measured in centimeters, and EFW is the estimated fetal weight in grams. The formula uses a logarithmic transformation (log base 10) of the weight, which means the actual weight is obtained by raising 10 to the power of the result.
Each measurement contributes differently to the estimate. The Abdominal Circumference appears in multiple terms because it is the strongest predictor of fetal weight — the abdomen contains the liver and subcutaneous fat, which are the most variable components of fetal mass. The Femur Length reflects skeletal growth and linear body size. The Head Circumference and Biparietal Diameter add information about cranial size, which helps refine the estimate, particularly in cases where the baby's body proportions differ from average.
The Hadlock four-parameter formula has been shown in numerous validation studies to estimate weight within 15% of actual birth weight in approximately 85% of cases. It performs best in the weight range of 1,000 to 3,500 grams and at gestational ages between 24 and 40 weeks. Below 24 weeks, the measurements can be less reliable due to the baby's small size and increased measurement variability.
Understanding Percentiles — SGA, AGA, and LGA
Once the estimated fetal weight is calculated, it is compared against reference data for the baby's gestational age to determine the percentile. This percentile is then used to classify the baby into one of three categories:
- SGA (Small for Gestational Age): Below the 10th percentile. This means the baby weighs less than 90% of other babies at the same gestational age. SGA does not automatically mean there is a problem — some babies are constitutionally small due to parental body size, ethnicity, or other normal factors. However, SGA can also be a sign of intrauterine growth restriction (IUGR), placental insufficiency, or other conditions that may require monitoring.
- AGA (Appropriate for Gestational Age): Between the 10th and 90th percentile. This is considered the normal range. The vast majority of healthy babies fall within this range. A baby at the 15th percentile is just as "normal" as a baby at the 85th percentile — the key is consistent growth along the baby's own growth curve.
- LGA (Large for Gestational Age): Above the 90th percentile. This means the baby weighs more than 90% of other babies at the same gestational age. LGA can be associated with maternal diabetes (gestational or pre-existing), maternal obesity, or genetic factors. LGA babies may face increased risk of birth injuries, shoulder dystocia, and neonatal hypoglycemia, so additional monitoring may be recommended.
The percentile is calculated using a normal distribution (bell curve) centered on the average weight for each gestational age. The standard deviation is approximately 15% of the mean weight at each gestational week. A z-score is first calculated as the difference between the estimated weight and the mean, divided by the standard deviation. This z-score is then converted to a percentile using the cumulative distribution function of the standard normal distribution.
It is worth noting that different growth charts may produce slightly different percentile results for the same weight. This is because the reference populations used to construct the charts differ. A baby that is at the 8th percentile on one chart might be at the 12th percentile on another. For consistency, it is best to use the same growth chart throughout pregnancy when tracking a baby's growth trajectory.
Fetal Weight by Gestational Age
The following table shows the approximate average (50th percentile) fetal weight at each gestational week, along with the 10th and 90th percentile boundaries. These values are based on large population studies and serve as general reference points. Individual variation is normal and expected.
| Gestational Week | 10th Percentile (g) | 50th Percentile (g) | 90th Percentile (g) |
|---|---|---|---|
| 14 | 79 | 93 | 107 |
| 16 | 124 | 146 | 168 |
| 18 | 189 | 222 | 255 |
| 20 | 281 | 331 | 381 |
| 22 | 406 | 478 | 550 |
| 24 | 565 | 665 | 765 |
| 26 | 767 | 902 | 1,037 |
| 28 | 980 | 1,153 | 1,326 |
| 30 | 1,325 | 1,559 | 1,793 |
| 32 | 1,616 | 1,901 | 2,186 |
| 34 | 2,003 | 2,357 | 2,711 |
| 36 | 2,391 | 2,813 | 3,235 |
| 38 | 2,704 | 3,181 | 3,658 |
| 40 | 2,943 | 3,462 | 3,981 |
| 42 | 3,132 | 3,685 | 4,238 |
These weights represent averages from large populations. Your baby's actual weight may differ and can still be perfectly healthy. Factors such as parental height and weight, ethnicity, sex of the baby, altitude, and maternal nutrition all influence fetal weight. Male babies tend to weigh slightly more than female babies at the same gestational age, and first-born babies tend to be slightly smaller than subsequent siblings.
It is also important to understand that fetal weight gain accelerates dramatically in the third trimester. Between weeks 28 and 40, the baby typically gains approximately 200–250 grams per week, compared to just 50–80 grams per week in the second trimester. This rapid weight gain is driven by fat deposition, which is critical for temperature regulation and energy storage after birth.
When to Be Concerned
While a single ultrasound measurement provides useful information, it is the pattern of growth over time that truly matters. There are several situations in which healthcare providers may express concern about fetal weight:
Weight Too Low (SGA / IUGR)
When a baby's estimated weight falls below the 10th percentile, the condition is classified as Small for Gestational Age (SGA). However, not all SGA babies have a problem. To distinguish between a baby that is constitutionally small (healthy but genetically predisposed to be smaller) and one that is growth-restricted (IUGR — Intrauterine Growth Restriction), doctors look at additional factors:
- Growth velocity: Is the baby's weight percentile dropping over time, or has it always been low? A stable trajectory at the 8th percentile is less concerning than a drop from the 40th to the 5th percentile.
- Doppler flow studies: Ultrasound Doppler measurements of blood flow through the umbilical artery and other fetal vessels can reveal whether the placenta is delivering adequate blood and oxygen. Abnormal Doppler patterns suggest placental insufficiency.
- Amniotic fluid volume: Reduced amniotic fluid (oligohydramnios) can accompany growth restriction and indicates reduced fetal urine output, which may signal compromised kidney perfusion.
- Maternal factors: Conditions such as preeclampsia, chronic hypertension, smoking, substance use, and severe malnutrition can cause growth restriction.
True IUGR affects approximately 3–7% of pregnancies and carries increased risks of stillbirth, preterm delivery, and neonatal complications. Management may include more frequent monitoring, antenatal steroids to accelerate lung maturity, and early delivery if the risks of remaining in utero outweigh the risks of prematurity.
Weight Too High (LGA / Macrosomia)
When estimated fetal weight exceeds the 90th percentile, the baby is classified as Large for Gestational Age (LGA). If the estimated weight exceeds 4,000 grams (some sources use 4,500 grams), the term macrosomia is used. Potential concerns include:
- Maternal diabetes: Uncontrolled glucose levels lead to excess fetal insulin production, which acts as a growth factor causing the baby to deposit excess fat and grow larger.
- Birth complications: LGA babies have higher rates of cesarean delivery, shoulder dystocia (where the shoulders become stuck during vaginal delivery), birth injuries, and postpartum hemorrhage.
- Neonatal hypoglycemia: Babies of diabetic mothers may experience low blood sugar after birth because they are no longer receiving excess glucose from the mother but continue to produce high levels of insulin.
If your baby is consistently measuring large, your healthcare provider may recommend glucose tolerance testing, more frequent ultrasounds, and discussions about delivery planning. In many cases, LGA is simply a reflection of genetic factors — taller, larger parents tend to have larger babies.
Frequently Asked Questions
How accurate is ultrasound for estimating fetal weight?
Ultrasound-based fetal weight estimation using the Hadlock formula is accurate to within 10–15% of the actual birth weight in most cases. This means that for a baby estimated at 3,000 grams, the actual weight could range from about 2,550 to 3,450 grams. Accuracy tends to decrease at the extremes of fetal weight (very small or very large babies) and in late pregnancy when the baby's position can make measurements more difficult. Despite these limitations, ultrasound remains the best available non-invasive method for estimating fetal weight.
What if my baby's percentile changed between ultrasounds?
Small fluctuations in percentile between ultrasound visits are completely normal and are often due to measurement variability rather than actual changes in growth rate. A shift of 10–15 percentile points between scans is within the expected range of measurement error. However, a consistent downward trend across multiple ultrasounds (for example, dropping from the 50th to the 20th to the 8th percentile over three consecutive scans) may indicate growth restriction and warrants further evaluation. Similarly, a rapid increase in percentile could suggest emerging gestational diabetes or other factors.
Does fetal sex affect the weight percentile?
Yes, male babies tend to weigh slightly more than female babies at the same gestational age, with the difference becoming more pronounced in the third trimester. By 40 weeks, male babies average approximately 100–150 grams heavier than female babies. Some growth charts are sex-specific, while others use combined data. The calculator on this page uses combined (non-sex-specific) reference values, which are adequate for general screening purposes. Your healthcare provider may use sex-specific charts for more precise assessment.
Can I use this calculator to predict my baby's birth weight?
This calculator estimates the baby's current weight based on the ultrasound measurements entered, not the weight at birth. Fetal weight gain varies considerably in the final weeks of pregnancy, so an estimate at 32 weeks cannot reliably predict birth weight at 40 weeks. However, if you enter measurements taken close to your due date (37–40 weeks), the estimate will be closer to the actual birth weight. Keep in mind the 10–15% margin of error inherent in all ultrasound weight estimates.
What should I do if my baby is classified as SGA?
If your baby is classified as Small for Gestational Age (below the 10th percentile), do not panic. Your healthcare provider will likely recommend additional testing to determine whether the baby is constitutionally small or experiencing growth restriction. This may include serial ultrasounds every 2–3 weeks to track growth velocity, Doppler studies to assess placental blood flow, and non-stress tests to monitor fetal heart rate patterns. In many cases, SGA babies are perfectly healthy and simply reflect normal variation in fetal size. Your provider will guide you on any necessary interventions based on the complete clinical picture.