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📘 About the Fetal Weight (EFW)
Ultrasound reports list the baby's measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Machines combine them into an estimated fetal weight (EFW) using Hadlock's formulas — this calculator does the same.
Add the gestational age to see the approximate percentile: below the 10th is called small for gestational age (SGA) and above the 90th large for gestational age (LGA). Scan estimates have a margin of about ±15%.
🧮 Formula
✏️ Worked examples
32-week scan: BPD 82, HC 296, AC 280, FL 62 mm
You get| Estimated fetal weight (EFW) | 1,928 g (1.93 kg · 4.25 lb) |
| Formula used | Hadlock 4 (BPD, HC, AC, FL) |
| Percentile for 32 weeks (approx.) | 46th percentile |
| Average weight at this stage | 1,953 g |
| Assessment | Appropriate for gestational age (10th–90th) |
| Typical margin of error | ±289 g (ultrasound estimates are within about ±15%) |
EFW 1,928 g (1.93 kg · 4.25 lb); 46th percentile.
Try this example →AC 330 mm, FL 72 mm at 37 weeks
You get| Estimated fetal weight (EFW) | 3,102 g (3.1 kg · 6.84 lb) |
| Formula used | Hadlock 2 (AC, FL) |
| Percentile for 37 weeks (approx.) | 58th percentile |
| Average weight at this stage | 3,028 g |
| Assessment | Appropriate for gestational age (10th–90th) |
| Typical margin of error | ±465 g (ultrasound estimates are within about ±15%) |
EFW 3,102 g (3.1 kg · 6.84 lb).
Try this example →❓ Frequently asked questions
How accurate is estimated fetal weight?
Usually within ±15% of the birth weight; accuracy is lower for very small or very large babies.
What is a normal fetal weight at 32 weeks?
About 1.95 kg on average (Hadlock 50th percentile), with a normal range of roughly 1.65–2.3 kg.
What do BPD, HC, AC and FL mean?
Biparietal diameter (head width), head circumference, abdominal circumference and femur (thigh bone) length.
My scan says the baby is small — what next?
Your doctor may repeat scans every 2–3 weeks and check blood flow (Doppler) to make sure the baby is growing well.