Comments :

The equation from the Hobbins/Shepard group still remains the most accurate equation for weight estimation. Equations generated from data containing the FL (eg. from Hadlock et al) tend to yield significantly more superior results in some studies but not in others. Look-up charts cannot be used with 3 parameters but programmable calculators can be useful for this purpose. 3-parameters equations are theoretically more sound and should be used to obtain the most accurate results over a wider range of fetal weights. The mean error at term should be within 250 grams.

Campbell’s equation using the AC alone is also accurate and this can definitely be recommended to be used in routine practice. The lookup is so much faster.

Estimation of fetal weight is by far one of the most important use of ultrasonic measurements in Obstetrics. It is also the most logical way of interpreting fetal growth basing on ultrasonic parameters. When the estimated weight is fitted onto an intrauterine weight normogram, it will at once become clear as to whether the fetus is growth retarded or not.

At times when charts are not at hand, one can try out this simple equation* :
Birthweight (in grams) = 1.4x BPD x FL x AC (all in cm) - 200.
The equation is more accurate in fetuses below 2.5 kg.

*Woo, J.S.K., Wan, C.W.: Weight estimation in the small for dates fetus - an evaluation of a simple equation containg the fetal femur length. Journal of Ultrasound in Medicine 5:453, 1986.

Joseph Woo.

Back to Obstetric ultrasound