What is estimated fetal weight?
Estimated fetal weight (EFW) is an estimate of the baby's weight in the womb, calculated from ultrasound measurements. Because the baby cannot be weighed directly, measurements of the head, abdomen and thigh bone are entered into mathematical formulas. Ultrasound reports usually show it in grams as "EFW".
Which measurements are used?
- BPD (biparietal diameter): the width of the head between the two parietal bones.
- HC (head circumference): the distance around the head.
- AC (abdominal circumference): the distance around the abdomen; it has the greatest effect on weight.
- FL (femur length): the length of the thigh bone.
The Hadlock formulas
This tool uses the Hadlock (1985) formulas, among the most widely used methods worldwide. Measurements are converted to centimetres and the base-10 logarithm of the weight is calculated. When all four measurements are entered, the formula is:
log10(EFW) = 1.3596 − 0.00386 × AC × FL + 0.0064 × HC + 0.00061 × BPD × AC + 0.0424 × AC + 0.174 × FL
Without head circumference, the Hadlock formula using BPD, AC and FL is applied; without any head measurement, the formula using AC and FL only. The result shows which formula was used.
What does the percentile mean?
If you also enter the gestational age, the estimated weight is compared with the in-utero weight reference published by Hadlock and colleagues in 1991. The 50th percentile, for example, means half of babies at the same week weigh less and half weigh more.
- Between the 10th and 90th percentile: the expected range.
- Below the 10th percentile: small for gestational age. The baby may be constitutionally small, but growth restriction may also need to be assessed.
- Above the 90th percentile: large for gestational age. Besides family build, causes such as gestational diabetes are considered.
Limitations
Ultrasound weight estimation is approximate and can differ from the actual birth weight by about 10-15% on average. The error is larger in very small or very large babies, twin pregnancies, low amniotic fluid or when imaging is difficult. The percentile depends on the gestational age having been set correctly by an early scan. The pattern of growth over several scans matters more than a single measurement.
When to see a doctor
Talk to your doctor if the result is below the 10th or above the 90th percentile, or if you notice reduced baby movements, bleeding, leaking fluid, severe headache or swelling. This tool is meant to help you understand the values in your report; decisions about your baby's growth, further tests and the timing of birth are made by your doctor together with the examination findings.
Sources
- Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985;151:333-337.
- Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology. 1991;181:129-133.