Estimated Fetal Weight at 21 Weeks Gestation
At 21 weeks gestation, the estimated fetal weight (EFW) is approximately 360-400 grams, based on standardized ultrasound biometric measurements using the Hadlock formula combining head circumference, abdominal circumference, and femur length.
How EFW is Determined
Ultrasonographic estimation of fetal weight is generated using regression equations that combine biometric measurements of the fetal biparietal diameter, head circumference (HC), abdominal circumference (AC), and femur length 1.
The most accurate approach uses:
- Hadlock formula incorporating HC, AC, and femur length - this is the recommended standard 2
- Formulas based on 3-4 fetal biometric indices are significantly more accurate than those using only 1-2 measurements 1
- The addition of biparietal diameter does not meaningfully increase accuracy 2
Expected Weight Range
Based on established fetal growth curves:
- At 20 weeks: approximately 300-370 grams (50th percentile around 368g) 3
- At 21 weeks: approximately 360-400 grams (interpolated from available data)
- At 22 weeks: approximately 430-480 grams
The normal variance is approximately ±12.7% (1 standard deviation) throughout gestation 4.
Clinical Context and Accuracy
Important caveats about EFW accuracy:
- Most formulas have an inherent imprecision of ±10% of actual fetal weight 2
- Accuracy depends heavily on image quality, proper technique, and standardized measurement planes 1, 2
- At 21 weeks, the fetus is still in the second trimester where growth velocity is rapid (approximately 200g per week gain occurs later from 27-38 weeks) 3
The EFW should be plotted on population-based reference charts (such as Hadlock curves) to generate a weight percentile for gestational age 1. This allows identification of fetuses that may be small-for-gestational-age (below 10th percentile) or growth restricted (below 3rd percentile with additional concerning features).
Quality Assurance Requirements
For accurate measurements at any gestational age including 21 weeks: