Ultrasonographic Parameters for Fetal Weight Prediction

Estimation of fetal weight (EFW: Estimated Fetal Weight) with ultrasound examination is a common practice in obstetrics and is important for planning the mode and timing of birth, especially in pregnancies at risk for altered fetal growth.

The conditions most commonly associated with impaired fetal growth are:

• IUGR: in these cases the ultrasound estimate of fetal weight is fundamental to decide the timing of delivery, since the neonatal outcome correlates positively with the fetal weight as well as with the flowmetry parameters
• fetal macrosomia (fetal weight greater than 4500 g) associated or not with gestational diabetes: in such cases an accurate estimation of the fetal weight is necessary and careful evaluation of any fetal-pelvic disproportion to decide both the timing of the birth that can be anticipated compared to at the physiological end of pregnancy is the mode of birth, vaginal or laparotomic in the event of a pelvic fetus disproportion.

In the 1970s the estimate of fetal weight was based on the measurement of the symphysis-fundus uterine distance (SFH) for which:

• SFH \<33 cm: predictive of a fetal weight \<3100 gr
• SFH> 34 cm: predictive of a fetal weight = o> 4000 gr.

At present, the fetal weight estimate is performed with biometric ultrasound parameters:

• DBP: biparietal diameter
• HC: head circumference
• AC: abdominal circumference
• FL: femur length There are standardized tables of variation of these parameters according to the gestational epoch for which the knowledge of the correct gestational epoch is fundamental for the correct interpretation of the data.

The fetal weight is calculated using mathematical formulas automatically obtained from ultrasound equipment of which the most used:

• Shepard's formula as a function of BPD and AC.
• formula of Campbell and Wilkin: it is in function of the abdominal circumference
• Hadlock formula: use the combination of different biometric parameters (BPD-HC-AC-FL).

Even if the guidelines provide for three ultrasound examinations for physiological pregnancies (one for each trimester of pregnancy), it is common practice to evaluate the biometric ultrasound parameters at the end of pregnancy and in any case when the patient is admitted for the delivery. It is obvious that in this time of pregnancy a closer correlation between biometric parameters and fetal weight is observed.

The problems related to the ultrasound estimation of fetal weight are represented by:

• significant intra-operator variability
• reduced accuracy for extremes of fetal weight (small or macrosomic fetuses)
• about 10% discrepancy between estimated fetal weight on an echographic basis according to Hadlock and actual weight of the newborn at birth.