Adverse events in the process of fetal growth can lead to consequences at later times and the extent of the damage depends on the nature, duration and severity. Two crucial determinants of fetal growth are the pregnancy duration and the fetal weight at a specific gestational age(1).
As weight is easy to measure at birth, it is used as an indicator of fetal growth, and is included in the comparative statistics on perinatal health improvements. There is a wide variety of normal birth weights within a specific population and between distinct populations. The factors that determine birth weight are not necessarily the same in different populations. It is a consensus that the maternal environment is extremely important for proper fetal growth and is influenced by genetic factors, biological potential and several environmental, fetal and placental regulators and modulating factors(2).
Among the parameters used in newborn evaluation, the neonatal birth weight is an important variable that predicts neonatal morbidity and mortality. The association between prematurity, low birth weight and neonatal mortality/morbidity is well established(3). This statement makes it clear that fetal weight calculation is an important factor in obstetric practice, as this is often decisive in continuing or interrupting high risk pregnancies.
Ultrasound, a technique that depends on equipment and specialized human resources, is the most important method for estimating fetal weight. However, even with adequate technology, the estimated fetal weight is less accurate in cases of very low birth weight and a macrosomic fetus(4). Several formulas were created by different authors for estimating fetal weight from biometric measures, which include biparietal diameter (BPD), head circumference (HC), femur length (FL) and abdominal circumference (AC). Hadlock
The formulas for estimating fetal weight were built by a regression analysis based on birth weight and biometric parameters at different gestational ages. Substantial bias occurs in the biometric parameters at each gestational age between different authors, and thus the value calculated as weight, using a given formula with different biometric authors, will give the same result, but when distributed on a normal curve it will occur at different percentile positions, creating a confusion factor about fetal growth normality(6).
In Brazil, Pedreira
The aim of this study was to use the Pedreira
We conducted an observational study using formulas and tables which were selected by their importance in the international literature. This study was approved by the Ethic Committee of Federal University of Santa Catarina (UFSC). For fetal estimation weight, we used the formula of Warsof
Authors and their respective estimated fetal weight formulas
Author | Formula |
---|---|
Warsof |
Log10(BW) = 0.144*BPD + 0.032*AC − 0.000111*AC*BPD2 − 1.599 |
Shepard |
Log10(BW) = 0.166*BPD + 0.046*AC − 0.002646*AC*BPD − 1.7492 |
Hadlock |
Log10(BW) = 1.3596 – 0.00386*AC*FL + 0.0064*HC + 0.0424*AC + 0174*FL + 0.00061*BPD*AC |
Hadlock |
Log10(BW) = 1.326 - 0.00326*AC*FL + 0.0107*HC + 0.00061*AC + 0.158*FL |
Furlan |
EFW = -8.277 + 2.146*BPD*AC*FL - 2.449*FL*BPD2 |
Stirnemann |
Log (EFW) = 5.084820 − 54.06633*(AC/100)3 − 95.80076*(AC/100)3*Log(AC/100) + 3.136370*(HC/100) |
The fetal weight calculation was found by mixing different formulas and different biometrics tables. The data were plotted in a standard Excel 2011 program table (Microsoft Corp., Redmond, WA, USA). Statistical analyses were performed using the mean relative error (standard weight – estimated weight), average absolute error ((standard weight – estimated weight)/standard weight) and the Pearson correlation coefficient (r) with dispersion graphics using 2011 Excel statistical function calculations.
The differences between the newborn weights and calculated estimated weight in the different tables and different biometric weight calculation formulas were studied by the average relative error and absolute error average.
The relative error: we observed that the formulas by Shepard
Formula/Biometry | Snijders and Nicolaides(13) | Papageorghiou |
Hadlock |
Kiserud |
---|---|---|---|---|
Hadlock |
0.01 | 0.01 | 0.01 | 0.01 |
Hadlock |
0.01 | 0.01 | 0.01 | 0.01 |
Warsof |
0.01 | 0.01 | 0.01 | 0.01 |
Shepard |
0.00 | 0.00 | 0.01 | 0.00 |
Furlan |
0.01 | 0.01 | 0.01 | 0.01 |
Stirnemann |
0.00 | 0.01 | 0.00 | 0.00 |
The absolute error: we observed that the formula by Furlan
Formula/Biometry | Snijders and Nicolaides(13) | Papageorghiou |
Hadlock |
Kiserud |
---|---|---|---|---|
Hadlock |
-8 | 3 | -10 | -10 |
Hadlock |
2 | 7 | -10 | -8 |
Warsof |
10 | 2 | 4 | -4 |
Shepard |
20 | -21 | -5 | -29 |
Furlan |
12 | 0 | -13 | -13 |
Stirnemann |
3 | 3 | -7 | -10 |
In the Pearson correlation coefficient (r) analyses, we noticed that the best correlation was not coincident in various formulas of the calculation of estimated weight, as shown in Tab. 4.
Formula/Biometry | Snijders and Nicolaides(13) | Papageorghiou |
Hadlock |
Kiserud |
---|---|---|---|---|
Hadlock |
0.997745838 | 0.995990300 | 0.996151154 | 0.994035237 |
Hadlock |
0.997523283 | 0.995894516 | 0.995958968 | 0.994255142 |
Warsof |
0.997627969 | 0.996870789 | 0.996436453 | 0.992502405 |
Shepard |
0.997432215 | 0.996879997 | 0.996473888 | 0.992268808 |
Furlan |
0.996447817 | 0.995783331 | 0.995528994 | 0.994303123 |
Stirnemann |
0.998840641 | 0.995005850 | 0.996191769 | 0.992300737 |
Figure 1, Fig. 2, Fig. 3 and Fig. 4 show the weight behavior by gestational age, linear dispersion and the best Pearson correlation coefficient. The absolute error analysis between biometric tables and weight calculation formulas in which we observed the lowest error was between Furlan
The estimated fetal weight by ultrasound at different gestational ages is extremely important for monitoring of macrosomia or fetal growth restriction. Lubchenco
The most commonly used formula for fetal estimated weight was developed by Hadlock
It was observed that using biometric tables and fetal estimated weight formulas from different authors produced different percentiles for the same biometric measurement. The fetal weight was the same, but this value assumed different positions in percentile distribution. It was observed that fetal weight estimated from one author’s formulas could be normal for a particular fetus, but above or below the normal percentile limits by another author. Thus, the use of fetal weight formulas and biometric tables constructed by different authors must be defined and used as the gold standard, so that comparisons can be made by different sonographers.
Pedreira
In summary, using the Pedreira