Fetal Weight Percentile Calculator
Estimate your baby’s weight percentile for gestational age using ultrasound biometry (Hadlock formula) — instantly see whether growth is tracking AGA, SGA, or LGA, with a visual growth-curve chart you can save or share with your provider.
Calculate Your Baby’s Weight Percentile
Enter ultrasound measurements or a known estimated fetal weight (EFW).
How the Fetal Weight Percentile Calculator Works
This calculator estimates your baby’s weight using the Hadlock formula, the most widely used and clinically validated method for calculating estimated fetal weight (EFW) from ultrasound measurements. It then compares that estimate against reference growth curves to determine where your baby falls relative to other fetuses at the same gestational age — expressed as a percentile.
What the Percentile Means
A percentile shows how your baby’s estimated weight compares with 100 typical fetuses at the same gestational age. For example, a baby in the 60th percentile is estimated to weigh more than 60 out of 100 babies at that same point in pregnancy — and less than the remaining 40. A percentile near 50 is considered average; it is not a “score” to optimize.
| Percentile Range | Clinical Classification | What It Generally Means |
|---|---|---|
| Below 3rd–10th | SGA (Small for Gestational Age) | Estimated weight is lower than expected; may warrant closer monitoring. |
| 10th – 90th | AGA (Appropriate for Gestational Age) | Estimated weight falls within the typical range for this stage of pregnancy. |
| Above 90th | LGA (Large for Gestational Age) | Estimated weight is higher than expected; provider may discuss further evaluation. |
Source: classifications based on ACOG Practice Bulletin guidance on fetal growth restriction and macrosomia.
Understanding the Four Ultrasound Measurements
Each measurement targets a different part of fetal anatomy. Together they’re combined into the Hadlock regression formula to produce a single weight estimate.
Diagram: simplified educational illustration of standard fetal biometric measurement planes (alt text: “fetal-ultrasound-biometry-bpd-hc-ac-fl-diagram”).
SGA, AGA & LGA: What Happens Next
If Your Baby Is Measuring Small (SGA)
A single low estimate isn’t automatically concerning — ultrasound has a real margin of error, and some babies are simply constitutionally smaller. Your provider will usually look at the growth trend across multiple scans, amniotic fluid levels, and umbilical artery Doppler flow before considering a diagnosis of fetal growth restriction (FGR).
If Your Baby Is Measuring Large (LGA)
Babies above the 90th percentile may be screened for maternal gestational diabetes if this hasn’t already been ruled out, since elevated blood sugar is one of several factors associated with higher fetal weight. Family size patterns and maternal BMI are also taken into account.
If Your Baby Is in the Typical Range (AGA)
Most pregnancies fall here. A single percentile reading is one data point among many your care team uses — it does not predict birth weight with certainty, nor does it diagnose any condition on its own.
Frequently Asked Questions
Medically Reviewed by Dr. Ruqaiya Khan
Dr. Khan reviewed this calculator’s methodology and reference ranges for clinical accuracy and alignment with current ACOG guidance on fetal growth assessment.
Sources & References
- Hadlock FP, et al. “Estimation of fetal weight with the use of head, body, and femur measurements.” American Journal of Obstetrics and Gynecology, 1985.
- American College of Obstetricians and Gynecologists (ACOG). Practice Bulletin: Fetal Growth Restriction.
- American College of Obstetricians and Gynecologists (ACOG). Practice Bulletin: Macrosomia.
- NHS. “Ultrasound scans in pregnancy.” nhs.uk
- Mayo Clinic. “Fetal ultrasound.” mayoclinic.org
Our fetal weight percentile calculator uses the Hadlock formula to estimate your baby’s growth trajectory. For a complete picture of your pregnancy timeline, pair these results with our Due Date Calculator, and review the ACOG guidance on fetal growth restriction for the clinical background behind weight percentile screening.