1Fetal Growth Chart by Week
The table below shows median estimated fetal weight (EFW) and crown-rump / crown-heel length by gestational week, based on WHO Multicentre Growth Reference data and Hadlock nomograms. Note that individual variation is normal — what matters clinically is consistent growth along a percentile curve.
| Week | Avg Weight | 10th %ile | 90th %ile | Length (cm) | Size Comparison |
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2How Is Fetal Growth Assessed?
Fetal growth is assessed primarily through obstetric ultrasound. The key measurements taken are:
- BPD (Biparietal Diameter): Width of the skull at the widest point. Most accurate for gestational dating before 20 weeks.
- HC (Head Circumference): Circumference of the fetal head. Reflects brain growth.
- AC (Abdominal Circumference): Circumference of the fetal abdomen at the level of the liver. The most sensitive indicator of fetal nutrition and growth restriction.
- FL (Femur Length): Length of the thigh bone. Reflects skeletal growth and long bone development.
- CRL (Crown-Rump Length): Used only in the first trimester (up to 14 weeks) to establish gestational age.
The Hadlock EFW Formula
The Hadlock four-parameter formula is the most widely used globally for estimating fetal weight from ultrasound biometry:
log₁₀(EFW) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD × AC) − 0.00386(AC × FL)
The result is in grams. The Hadlock formula has a systematic error (standard deviation) of approximately ±7.5–15% of actual birth weight — meaning an EFW of 2,500 g could reflect a real weight of 2,125–2,875 g. This inherent imprecision is why growth trends over serial scans matter more than any single measurement.
SGA, AGA, and LGA Explained
Growth is classified by comparing EFW or birth weight to a reference population at the same gestational age:
- SGA (Small-for-gestational-age): EFW <10th percentile. Severe SGA is <3rd percentile and may indicate fetal growth restriction (FGR).
- AGA (Appropriate-for-gestational-age): EFW 10th–90th percentile. Normal range.
- LGA (Large-for-gestational-age): EFW >90th percentile. Associated with gestational diabetes, macrosomia, and birth complications.
It is important to note that some SGA babies are constitutionally small (appropriate for their genetic potential) and are not at risk, while others have pathological growth restriction requiring monitoring. Your obstetrician interprets these findings alongside Doppler studies, amniotic fluid index (AFI), and clinical history.
3References
Last medical review: May 2025 · RxMedCalc Clinical Editorial Team.
4Frequently asked questions
What is normal fetal weight at 20 weeks?
At 20 weeks, the average estimated fetal weight (EFW) is approximately 300 g (10.6 oz). The normal range (10th to 90th percentile) spans roughly 240–370 g.
What is considered a large-for-gestational-age baby?
A baby is considered large-for-gestational-age (LGA) when their estimated or actual weight is above the 90th percentile for their gestational age. This is associated with gestational diabetes and macrosomia.
What is small-for-gestational-age (SGA)?
Small-for-gestational-age (SGA) refers to a baby whose estimated or birth weight is below the 10th percentile for their gestational age. Severely SGA is defined as below the 3rd percentile and may indicate fetal growth restriction (FGR).
How is estimated fetal weight calculated on ultrasound?
The most common formula is the Hadlock formula, which uses a combination of ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The Hadlock 4-parameter formula has a standard error of approximately ±15% of actual birth weight.
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.