Abstract
Objectives:
To validate new references charts and equations for fetal biometry in an Italian unselected population.
Methods:
A cross-sectional study involving 4896 women with singleton viable pregnancies, at Artemisia Fetal Maternal Medical Centre between May 2009 and December 2009. Each woman was scanned only once, between 14+0 and 40+0 weeks of gestation. The fetal standard biometric measurements were recorded. For each parameter, regression models were fitted to estimate the percentile at each gestational age. In order to be compared to other reference equations, the fetal biometric measurements at each gestational age were expressed as Z-scores.
Results:
New fetal charts and references equations for Italian population were developed according to the recommend multistep statistical procedure.
Conclusion:
To our knowledge this is the first Italian study with the largest sample size ever reported in the literature. In addiction, our newer charts of reference centiles for fetal biometric measurements are useful in the obstetrical clinical practice for the Italian population.
Introduction
An appropriate evaluation of fetal biometry represents the cornerstone in the fetal growth assessment giving that an abnormal growth may be associated with an adverse perinatal outcome and may require a specific obstetrical care (1).
Many variables affect fetal growth (2, 3, 4) including physiological and pathological changes, such as maternal height and weight, drug or tobacco exposure, fetal sex, ethnicity (5), genetic syndromes, congenital anomalies and placental failure. Therefore, each particular population or ethnic group should have their own reference charts for the different fetal anthropometrical variables in order to provide the most accurate fetal assessment. Furthermore, fetal nomograms need to be revised regularly as fetus is growing up in the last decades (6, 7) and need to be constructed in accordance with the recommended method of analysis (8, 9). In fact, the choice of a standard approach is crucial, because inaccurate centiles obtained from an inferior method may mislead the obstetrician as to the true state of health of development of the fetus and increase the chance of suboptimal clinical care. The World Health Organization recommends the expression of measurements as Z-scores in order to allow relevant statistical analysis (10, 11).
The aim of our study is to develop new reference charts and equations for fetal standard biometry between 14 and 40 weeks of gestation in a cohort of healthy Italian women with low-risk singleton viable pregnancies.
Methods
The study included a population of 4896 Italian women undergoing ultrasound examination between the 14th and 40th weeks of gestation at Artemisia Fetal Maternal Medical Centre, between May 2009 and December 2009. Inclusion criteria were singleton pregnancies in Italian women with low-risk pregnancy. Exclusion criteria were multiple pregnancy, abnormal karyotype, congenital malformations, gestational or maternal diseases (such as hypertension and diabetes mellitus), no firsttrimester dating based on crown-rump length (CRL). Cases with low birth weight, preterm delivery or other prenatal complications were not excluded. Gestational age was based on the last menstrual period and in all cases adjusted according to the CRL measured in the first trimester ultrasound. Every fetus was measured only once for the study. All measurements were performed by 5 different obstetric sonographers (MG, PC, FP, MLM, RV) using the following equipments: GE Voluson 730, GE E8 and GE Voluson 730 Pro (General Electric Healthcare, USA).
Measurements of the biparietal diameter (BPD) and head circumference (HC) were obtained from a transverse axial plane of the fetal head showing a central midline echo broken in the anterior third by the cavum septum pellucidi and demonstrating the anterior and posterior horns of the lateral ventricle. The BPD was measured from the outer margin of the proximal skull to the inner margin of the distal skull. The HC was measured fitting a computer-generated ellipse to include the outer edges of the calvarial margins of the fetal skull.
The abdominal circumference (AC) was measured fitting a computer-generated ellipse through a transverse section of the fetal abdomen at the level of the stomach and bifurcation of the main portal vein into its right and left branches. The femur length (FL) was measured in a longitudinal scan where the whole femural diaphysis was seen almost parallel to the transducer and measured from the greater trochanter to the lateral condyle. In the third trimester, particular care was taken not to include the ephiphysis.
Statistical analysis was performed using SPSS 16.0 and STATISTICA 7.0. The data were analyzed as recommended by Altman and Royston (8, 9).
The normality of measurements at each week of gestation was assessed using the Kolmogorov–Smirnov test. Given the large sample size, statistically significant nonnormality was accepted unless the normal plot showed clear deviation from a straight line. In order to obtain normal ranges for fetal measurements, a multistep procedure based on regression model has been used, according to the recommended methodology.
A centile curve is calculated using the following formula: Centile = M + K x SD
where K is the corresponding centile of the standard Gaussian distribution, M is the mean and SD is the standard deviation of the mean of the fetal measurements for each gestational age.
The mean is estimated by the fitted values from an appropriate polynomial regression curve of the measurement of interest on gestational age. A cubic polynomial model was used (y = a + b × GA + c × GA2 + d × GA3) unless the R2 statistic and/or the fitted curve was not satisfactory.
The standard deviation is obtained through the following steps: the residuals were calculated as the absolute value of the differences between the fitted curve and the original data for each patient and then multiplied by a corrective constant equal to (=1.253). These were finally regressed on gestational ages by using a simple linear equation (y = a + b x GA). The fitted values give the age-specific SD estimates.
The standard deviation is obtained through the following steps: the residuals were calculated as the absolute value of the differences between the fitted curve and the original data for each patient and then multiplied by a corrective constant equal to (=1.253). These were finally regressed on gestational ages by using a simple linear equation (y = a + b x GA). The fitted values give the age-specific SD estimates.
Finally, these predictive mean and SD equations allow calculating any required centile, replacing the value in the centile formula. The constant K may be ±0.674 for the determination of the 25 th and 75 th centiles, ±1.282 for the 10th and 90th centiles, ±1.645 for the 5th and 95th centiles, ±1.96 for the 2.5 th and 97.5 th centiles, ±2.33 for 1st and 99 th centiles, ±2.58 for 0.5th and 99.5 th centiles.
In order to compare our new reference equations with previously published ones, we calculated Z-scores of the median, 5th and 95th percentiles, according the following formula:
Z-score = (XGA-MGA)/SDGA
where XGA is the measured value at a known gestational age (GA), MGA is the mean value obtained with the reference equation used at this GA, and SDGA is the SD associated with the mean value at this GA obtained with the reference equation. Results were presented graphically across GA.
Results
Full biometric measurements (BPD, HC, AC, FL) were obtained for 4896 fetuses. The best-fitted regression model describing the relationship between biometric parameters and GA was the cubic model. Data analysis showed that neither the use of fractional polynomials nor the logarithmic transformation improved the fitting of the curves. The corresponding regression equations for the mean are as follows:
BPDmean=12.707 - (0.771 x GA) + (0.186 x GA2) - (0.00291 x GA3) (R2=96.6)
HCmean=18.841 + (0.506 x GA) + (0.576 x GA2) - (0.00973 x GA3) (R2=97.4)
ACmean= -40.349 + (7.209 x GA) + (0.214 x GA2) - (0.00383 x GA3) (R2=95.2)
FLmean=-32.045 + (3.597 x GA) - (0.00917 x GA2) - (0.000353 x GA3) (R2=97.2)
SDs across GA were fitted using a simple linear model. Fits for SDs were as follows:
BPDSD= 1.174 + 0.069 x GA
HCSD= 1.795 + 0.295 x GA
ACSD= -1.447 + 0.542 x GA
FLSD= 0.676 + 0.061 x GA
Tables 1-4 show the centile charts with 0.5th, 1 th, 2.5 th, 5 th, 10 th, 50 th, 90 th, 95 th, 97.5 th, 99 th, 99. th centiles fitted for all biometric parameters. Figure 1 illustrates the goodness of fit of our model by showing the raw data for the main measurements in obstetrical practice with the fitted 5 th, 50 th and 95 th centiles. We graphically compared our Zscores to the ones derived by the 5 th, 50 th and 95 th percentiles calculated for the main biometric parameters of two previously published sets of reference equations (12, 13) (Figure 2).
Table 1 - .
Fitted centiles of biparietal diameter (BPD) (mm)
| GA | 0,5° | 1° | 2,5° | 5° | 10° | 25° | 50° | 75° | 90° | 95° | 97,5° | 99° | 99,5° |
| 14 | 24,9 | 25,4 | 26,2 | 26,9 | 27,6 | 28,9 | 30,4 | 31,8 | 33,1 | 33,9 | 34,6 | 35,4 | 35,9 |
| 15 | 27,5 | 28,0 | 28,8 | 29,5 | 30,3 | 31,7 | 33,2 | 34,7 | 36,0 | 36,8 | 37,5 | 38,3 | 38,9 |
| 16 | 30,2 | 30,8 | 31,6 | 32,3 | 33,1 | 34,5 | 36,1 | 37,6 | 39,0 | 39,8 | 40,5 | 41,4 | 41,9 |
| 17 | 33,0 | 33,6 | 34,5 | 35,2 | 36,0 | 37,5 | 39,1 | 40,6 | 42,1 | 42,9 | 43,7 | 44,5 | 45,1 |
| 18 | 35,9 | 36,5 | 37,4 | 38,1 | 39,0 | 40,5 | 42,1 | 43,8 | 45,2 | 46,1 | 46,9 | 47,8 | 48,4 |
| 19 | 38,8 | 39,5 | 40,4 | 41,2 | 42,1 | 43,6 | 45,2 | 46,9 | 48,4 | 49,3 | 50,1 | 51,0 | 51,7 |
| 20 | 41,8 | 42,5 | 43,4 | 44,2 | 45,1 | 46,7 | 48,4 | 50,1 | 51,7 | 52,6 | 53,4 | 54,4 | 55,0 |
| 21 | 44,8 | 45,5 | 46,5 | 47,3 | 48,2 | 49,8 | 51,6 | 53,4 | 55,0 | 55,9 | 56,7 | 57,7 | 58,4 |
| 22 | 47,8 | 48,5 | 49,5 | 50,4 | 51,3 | 53,0 | 54,8 | 56,6 | 58,2 | 59,2 | 60,1 | 61,1 | 61,7 |
| 23 | 50,8 | 51,5 | 52,6 | 53,4 | 54,4 | 56,1 | 58,0 | 59,8 | 61,5 | 62,5 | 63,4 | 64,4 | 65,1 |
| 24 | 53,8 | 54,5 | 55,6 | 56,5 | 57,5 | 59,2 | 61,1 | 63,0 | 64,7 | 65,8 | 66,7 | 67,7 | 68,4 |
| 25 | 56,7 | 57,5 | 58,5 | 59,4 | 60,5 | 62,3 | 64,2 | 66,2 | 67,9 | 69,0 | 69,9 | 71,0 | 71,7 |
| 26 | 59,6 | 60,3 | 61,4 | 62,4 | 63,4 | 65,3 | 67,3 | 69,3 | 71,1 | 72,1 | 73,1 | 74,2 | 74,9 |
| 27 | 62,4 | 63,1 | 64,3 | 65,2 | 66,3 | 68,2 | 70,2 | 72,3 | 74,1 | 75,2 | 76,2 | 77,3 | 78,0 |
| 28 | 65,0 | 65,8 | 67,0 | 68,0 | 69,1 | 71,0 | 73,1 | 75,2 | 77,0 | 78,2 | 79,2 | 80,3 | 81,1 |
| 29 | 67,6 | 68,4 | 69,6 | 70,6 | 71,7 | 73,7 | 75,8 | 77,9 | 79,9 | 81,0 | 82,0 | 83,2 | 84,0 |
| 30 | 70,0 | 70,8 | 72,0 | 73,1 | 74,2 | 76,2 | 78,4 | 80,6 | 82,6 | 83,7 | 84,8 | 86,0 | 86,8 |
| 31 | 72,3 | 73,1 | 74,4 | 75,4 | 76,6 | 78,6 | 80,9 | 83,1 | 85,1 | 86,3 | 87,4 | 88,6 | 89,4 |
| 32 | 74,4 | 75,3 | 76,5 | 77,6 | 78,8 | 80,9 | 83,1 | 85,4 | 87,5 | 88,7 | 89,8 | 91,0 | 91,9 |
| 33 | 76,3 | 77,2 | 78,5 | 79,6 | 80,8 | 82,9 | 85,2 | 87,6 | 89,7 | 90,9 | 92,0 | 93,3 | 94,1 |
| 34 | 78,1 | 78,9 | 80,2 | 81,3 | 82,6 | 84,8 | 87,1 | 89,5 | 91,6 | 92,9 | 94,0 | 95,3 | 96,2 |
| 35 | 79,5 | 80,4 | 81,8 | 82,9 | 84,2 | 86,4 | 88,8 | 91,2 | 93,4 | 94,7 | 95,8 | 97,2 | 98,1 |
| 36 | 80,8 | 81,7 | 83,1 | 84,2 | 85,5 | 87,8 | 90,2 | 92,7 | 94,9 | 96,3 | 97,4 | 98,8 | 99,7 |
| 37 | 81,8 | 82,7 | 84,1 | 85,3 | 86,6 | 88,9 | 91,4 | 93,9 | 96,2 | 97,5 | 98,7 | 100,1 | 101,0 |
| 38 | 82,5 | 83,5 | 84,9 | 86,1 | 87,4 | 89,8 | 92,3 | 94,9 | 97,2 | 98,6 | 99,8 | 101,2 | 102,1 |
| 39 | 83,0 | 83,9 | 85,4 | 86,6 | 88,0 | 90,3 | 92,9 | 95,5 | 97,9 | 99,3 | 100,5 | 101,9 | 102,9 |
| 40 | 83,1 | 84,1 | 85,5 | 86,8 | 88,2 | 90,6 | 93,2 | 95,9 | 98,3 | 99,7 | 100,9 | 102,4 | 103,4 |
| 41 | 82,9 | 83,9 | 85,4 | 86,6 | 88,1 | 90,5 | 93,2 | 95,9 | 98,3 | 99,8 | 101,0 | 102,5 | 103,5 |
Table 4 - .
Fitted centiles of femur length (FL) (mm)
| GA | 0,5° | 1° | 2,5° | 5° | 10° | 25° | 50° | 75° | 90° | 95° | 97,5° | 99° | 99,5° |
| 14 | 11,6 | 12,0 | 12,5 | 13,0 | 13,6 | 14,5 | 15,5 | 16,6 | 17,5 | 18,1 | 18,5 | 19,1 | 19,5 |
| 15 | 14,6 | 14,9 | 15,5 | 16,0 | 16,6 | 17,6 | 18,7 | 19,7 | 20,7 | 21,3 | 21,8 | 22,4 | 22,8 |
| 16 | 17,5 | 17,9 | 18,5 | 19,0 | 19,6 | 20,6 | 21,7 | 22,8 | 23,8 | 24,4 | 25,0 | 25,6 | 26,0 |
| 17 | 20,3 | 20,7 | 21,4 | 21,9 | 22,5 | 23,6 | 24,7 | 25,9 | 26,9 | 27,5 | 28,1 | 28,7 | 29,1 |
| 18 | 23,1 | 23,5 | 24,2 | 24,8 | 25,4 | 26,5 | 27,7 | 28,9 | 29,9 | 30,6 | 31,1 | 31,8 | 32,2 |
| 19 | 25,8 | 26,3 | 27,0 | 27,5 | 28,2 | 29,3 | 30,6 | 31,8 | 32,9 | 33,6 | 34,2 | 34,8 | 35,3 |
| 20 | 28,5 | 29,0 | 29,7 | 30,3 | 31,0 | 32,1 | 33,4 | 34,7 | 35,8 | 36,5 | 37,1 | 37,8 | 38,3 |
| 21 | 31,1 | 31,6 | 32,3 | 33,0 | 33,7 | 34,9 | 36,2 | 37,5 | 38,7 | 39,4 | 40,0 | 40,7 | 41,2 |
| 22 | 33,7 | 34,2 | 34,9 | 35,6 | 36,3 | 37,5 | 38,9 | 40,3 | 41,5 | 42,2 | 42,8 | 43,6 | 44,1 |
| 23 | 36,2 | 36,7 | 37,5 | 38,1 | 38,9 | 40,1 | 41,5 | 42,9 | 44,2 | 45,0 | 45,6 | 46,4 | 46,9 |
| 24 | 38,6 | 39,1 | 39,9 | 40,6 | 41,4 | 42,7 | 44,1 | 45,6 | 46,9 | 47,6 | 48,3 | 49,1 | 49,6 |
| 25 | 41,0 | 41,5 | 42,3 | 43,0 | 43,8 | 45,1 | 46,6 | 48,1 | 49,5 | 50,3 | 50,9 | 51,8 | 52,3 |
| 26 | 43,2 | 43,8 | 44,6 | 45,4 | 46,2 | 47,5 | 49,1 | 50,6 | 52,0 | 52,8 | 53,5 | 54,3 | 54,9 |
| 27 | 45,4 | 46,0 | 46,9 | 47,6 | 48,5 | 49,9 | 51,4 | 53,0 | 54,4 | 55,3 | 56,0 | 56,9 | 57,4 |
| 28 | 47,6 | 48,2 | 49,1 | 49,8 | 50,7 | 52,1 | 53,7 | 55,3 | 56,8 | 57,7 | 58,4 | 59,3 | 59,9 |
| 29 | 49,6 | 50,2 | 51,2 | 51,9 | 52,8 | 54,3 | 55,9 | 57,6 | 59,1 | 60,0 | 60,7 | 61,6 | 62,3 |
| 30 | 51,6 | 52,2 | 53,2 | 54,0 | 54,9 | 56,4 | 58,1 | 59,8 | 61,3 | 62,2 | 63,0 | 63,9 | 64,5 |
| 31 | 53,5 | 54,2 | 55,1 | 55,9 | 56,8 | 58,4 | 60,1 | 61,9 | 63,4 | 64,4 | 65,2 | 66,1 | 66,8 |
| 32 | 55,3 | 56,0 | 57,0 | 57,8 | 58,7 | 60,3 | 62,1 | 63,9 | 65,5 | 66,4 | 67,3 | 68,2 | 68,9 |
| 33 | 57,0 | 57,7 | 58,7 | 59,6 | 60,5 | 62,2 | 64,0 | 65,8 | 67,4 | 68,4 | 69,3 | 70,2 | 70,9 |
| 34 | 58,7 | 59,4 | 60,4 | 61,3 | 62,3 | 63,9 | 65,8 | 67,6 | 69,3 | 70,3 | 71,2 | 72,2 | 72,9 |
| 35 | 60,2 | 60,9 | 62,0 | 62,9 | 63,9 | 65,6 | 67,5 | 69,4 | 71,1 | 72,1 | 73,0 | 74,0 | 74,7 |
| 36 | 61,7 | 62,4 | 63,5 | 64,4 | 65,4 | 67,2 | 69,1 | 71,0 | 72,8 | 73,8 | 74,7 | 75,8 | 76,5 |
| 37 | 63,0 | 63,8 | 64,9 | 65,8 | 66,8 | 68,6 | 70,6 | 72,6 | 74,4 | 75,4 | 76,4 | 77,4 | 78,2 |
| 38 | 64,3 | 65,1 | 66,2 | 67,1 | 68,2 | 70,0 | 72,0 | 74,0 | 75,9 | 77,0 | 77,9 | 79,0 | 79,8 |
| 39 | 65,5 | 66,2 | 67,4 | 68,3 | 69,4 | 71,3 | 73,4 | 75,4 | 77,3 | 78,4 | 79,3 | 80,5 | 81,2 |
| 40 | 66,5 | 67,3 | 68,5 | 69,4 | 70,6 | 72,5 | 74,6 | 76,7 | 78,6 | 79,7 | 80,7 | 81,8 | 82,6 |
| 41 | 67,5 | 68,3 | 69,5 | 70,5 | 71,6 | 73,5 | 75,7 | 77,8 | 79,8 | 80,9 | 81,9 | 83,1 | 83,9 |
Figure 1.
Fitted 5th , 50th and 95th centile curves superimposed on the raw data for biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Dashed lines represent the 5th and 95th centiles and the thick central one the 50th centile.
Figure 2.
Comparison of our new equations with Salomon (solid lines) and Snijders (dashed lines) references for biparietal diameter (a), head circumference (b), abdominal circumference (c) and femur length (d). Dotted gridlines represent the expected Z-scores for 5th, 50th and 95th centiles, i.e. -1.645, 0 and 1.645 respectively.
Discussion
The need for reference curves for a specific population arises from the observation that fetal growth depends on many factors, including ethnicity. In fact in literature a variety of chart references are described, varying from different populations. On the other hand, there has been an increasing value of fetal biometric parameters, thus the needing of updated chart references that reflect the trend of fetal growth. The first statistical models developed for Italian fetuses date back to 1987, including biparietal diameter, head circumference and abdomen circumference from 1,426 healthy fetuses (14). More complete
Italian fetal size charts will be published almost 20 years later, including head, abdomen and long bones measurements, although the number of fetuses was only 626 and the Authors did not specify the formula for calculating the standard deviation, letting the comparison with their curves be impossible (15).
The construction of reference interval for fetal variables is crucial as inaccurate centiles produced by inferior methods may mislead an abnormal growth that may be associated with an adverse perinatal outcome and require a specific obstetrical care. We followed the method described by Altman and Chitty and Royston and subsequently confirmed by Royston, in which they claim the need of an approach based on statistical techniques and the term “normal range” should be substituted by “reference interval” since the former implies that any observation outside the range indicates an “abnormality”, which is not necessarily the case.
Table 2 - .
Fitted centiles of head circumference (HC) (mm)
| GA | 0,5° | 1° | 2,5° | 5° | 10° | 25° | 50° | 75° | 90° | 95° | 97,5° | 99° | 99,5° |
| 14 | 97 | 98 | 101 | 102 | 105 | 108 | 112 | 116 | 120 | 122 | 124 | 126 | 127 |
| 15 | 107 | 109 | 111 | 113 | 115 | 119 | 123 | 127 | 131 | 133 | 135 | 138 | 139 |
| 16 | 118 | 119 | 122 | 124 | 126 | 130 | 135 | 139 | 143 | 145 | 147 | 150 | 151 |
| 17 | 129 | 130 | 133 | 135 | 137 | 142 | 146 | 151 | 155 | 157 | 159 | 162 | 164 |
| 18 | 139 | 141 | 144 | 146 | 149 | 153 | 158 | 163 | 167 | 170 | 172 | 174 | 176 |
| 19 | 151 | 152 | 155 | 157 | 160 | 165 | 170 | 175 | 179 | 182 | 184 | 187 | 189 |
| 20 | 162 | 164 | 166 | 169 | 172 | 176 | 182 | 187 | 191 | 194 | 197 | 199 | 201 |
| 21 | 173 | 175 | 178 | 180 | 183 | 188 | 193 | 199 | 204 | 207 | 209 | 212 | 214 |
| 22 | 184 | 186 | 189 | 192 | 195 | 200 | 205 | 211 | 216 | 219 | 221 | 224 | 227 |
| 23 | 195 | 197 | 200 | 203 | 206 | 211 | 217 | 223 | 228 | 231 | 234 | 237 | 239 |
| 24 | 205 | 208 | 211 | 214 | 217 | 222 | 228 | 234 | 240 | 243 | 246 | 249 | 251 |
| 25 | 216 | 218 | 221 | 224 | 228 | 233 | 239 | 246 | 251 | 255 | 257 | 261 | 263 |
| 26 | 226 | 228 | 232 | 235 | 238 | 244 | 250 | 257 | 262 | 266 | 269 | 272 | 275 |
| 27 | 236 | 238 | 242 | 245 | 248 | 254 | 261 | 267 | 273 | 277 | 280 | 284 | 286 |
| 28 | 245 | 248 | 251 | 254 | 258 | 264 | 271 | 278 | 284 | 288 | 291 | 294 | 297 |
| 29 | 254 | 257 | 260 | 264 | 267 | 274 | 281 | 288 | 294 | 298 | 301 | 305 | 307 |
| 30 | 262 | 265 | 269 | 272 | 276 | 283 | 290 | 297 | 303 | 307 | 311 | 315 | 317 |
| 31 | 270 | 273 | 277 | 280 | 284 | 291 | 298 | 306 | 312 | 316 | 320 | 324 | 326 |
| 32 | 277 | 280 | 284 | 288 | 292 | 298 | 306 | 314 | 320 | 325 | 328 | 332 | 335 |
| 33 | 283 | 286 | 291 | 294 | 298 | 305 | 313 | 321 | 328 | 332 | 336 | 340 | 343 |
| 34 | 289 | 292 | 296 | 300 | 304 | 312 | 319 | 327 | 335 | 339 | 343 | 347 | 350 |
| 35 | 294 | 297 | 301 | 305 | 309 | 317 | 325 | 333 | 341 | 345 | 349 | 353 | 356 |
| 36 | 298 | 301 | 305 | 309 | 314 | 321 | 330 | 338 | 346 | 350 | 354 | 359 | 362 |
| 37 | 300 | 304 | 308 | 312 | 317 | 325 | 333 | 342 | 350 | 354 | 358 | 363 | 366 |
| 38 | 302 | 306 | 310 | 315 | 319 | 327 | 336 | 345 | 353 | 357 | 361 | 366 | 369 |
| 39 | 303 | 307 | 311 | 316 | 320 | 329 | 337 | 346 | 355 | 359 | 364 | 368 | 372 |
| 40 | 303 | 306 | 311 | 316 | 321 | 329 | 338 | 347 | 355 | 360 | 365 | 370 | 373 |
| 41 | 301 | 305 | 310 | 314 | 319 | 328 | 337 | 347 | 355 | 360 | 364 | 370 | 373 |
Table 3 - .
Fitted centiles of abdominal circumference (AC) (mm)
| GA | 0,5° | 1° | 2,5° | 5° | 10° | 25° | 50° | 75° | 90° | 95° | 97,5° | 99° | 99,5° |
| 14 | 76 | 78 | 80 | 82 | 84 | 88 | 92 | 96 | 100 | 102 | 104 | 106 | 108 |
| 15 | 86 | 87 | 90 | 92 | 94 | 99 | 103 | 108 | 112 | 114 | 116 | 119 | 120 |
| 16 | 95 | 97 | 100 | 102 | 105 | 109 | 114 | 119 | 123 | 126 | 128 | 131 | 133 |
| 17 | 105 | 107 | 110 | 112 | 115 | 120 | 125 | 130 | 135 | 138 | 140 | 143 | 145 |
| 18 | 115 | 117 | 120 | 123 | 126 | 131 | 136 | 142 | 147 | 150 | 153 | 156 | 158 |
| 19 | 125 | 127 | 130 | 133 | 136 | 142 | 148 | 154 | 159 | 162 | 165 | 168 | 170 |
| 20 | 135 | 137 | 140 | 143 | 147 | 152 | 159 | 165 | 171 | 174 | 177 | 181 | 183 |
| 21 | 144 | 147 | 150 | 154 | 157 | 163 | 170 | 177 | 183 | 186 | 189 | 193 | 196 |
| 22 | 154 | 157 | 161 | 164 | 168 | 174 | 181 | 188 | 194 | 198 | 202 | 205 | 208 |
| 23 | 164 | 166 | 170 | 174 | 178 | 185 | 192 | 199 | 206 | 210 | 214 | 218 | 220 |
| 24 | 173 | 176 | 180 | 184 | 188 | 195 | 203 | 211 | 218 | 222 | 226 | 230 | 233 |
| 25 | 183 | 186 | 190 | 194 | 198 | 206 | 214 | 222 | 229 | 234 | 238 | 242 | 245 |
| 26 | 192 | 195 | 200 | 204 | 208 | 216 | 224 | 233 | 241 | 245 | 249 | 254 | 257 |
| 27 | 201 | 204 | 209 | 213 | 218 | 226 | 235 | 244 | 252 | 257 | 261 | 266 | 269 |
| 28 | 210 | 213 | 218 | 223 | 228 | 236 | 245 | 254 | 263 | 268 | 272 | 277 | 281 |
| 29 | 218 | 222 | 227 | 232 | 237 | 246 | 255 | 265 | 274 | 279 | 283 | 289 | 292 |
| 30 | 227 | 231 | 236 | 241 | 246 | 255 | 265 | 275 | 284 | 289 | 294 | 300 | 303 |
| 31 | 235 | 239 | 245 | 249 | 255 | 264 | 275 | 285 | 294 | 300 | 305 | 310 | 314 |
| 32 | 243 | 247 | 253 | 258 | 264 | 273 | 284 | 295 | 304 | 310 | 315 | 321 | 325 |
| 33 | 251 | 255 | 261 | 266 | 272 | 282 | 293 | 304 | 314 | 320 | 325 | 331 | 335 |
| 34 | 258 | 262 | 268 | 274 | 280 | 290 | 302 | 313 | 323 | 330 | 335 | 341 | 345 |
| 35 | 265 | 269 | 276 | 281 | 287 | 298 | 310 | 322 | 332 | 339 | 344 | 351 | 355 |
| 36 | 271 | 276 | 282 | 288 | 295 | 306 | 318 | 330 | 341 | 348 | 353 | 360 | 364 |
| 37 | 277 | 282 | 289 | 295 | 301 | 313 | 325 | 338 | 349 | 356 | 362 | 369 | 373 |
| 38 | 283 | 288 | 295 | 301 | 308 | 320 | 332 | 345 | 357 | 364 | 370 | 377 | 382 |
| 39 | 288 | 293 | 301 | 307 | 314 | 326 | 339 | 352 | 364 | 371 | 378 | 385 | 390 |
| 40 | 293 | 298 | 306 | 312 | 319 | 332 | 345 | 359 | 371 | 379 | 385 | 392 | 397 |
| 41 | 297 | 303 | 310 | 317 | 324 | 337 | 351 | 365 | 378 | 385 | 392 | 399 | 405 |
The representation of the fetal equations into Z-scores, as suggested by Salomon, lead to evaluate the effect of the reference curve chosen on the quality of screening for growth abnormalities. Indeed, knowing only the mean and the standard deviation, the Z-score allows a comparison between different curves. In our series, the graphical representation highlights that Italian fetuses have smaller HC and AC than English and French fetuses. This may suggest that Italian fetuses are smaller than other populations or that it may arise from the different methods used by Salomon in the measurement of HC and AC, which were manually calculated from BPD and FOD and the two maximum abdominal diameters, respectively. Finally Italian fetuses seem to have much smaller femurs than the English, at least until the beginning of the third trimester, but similar length than French femurs.
Conclusion
Our new Italian reference interval charts for fetal biometry and reference equations have clinical relevance since they provide sonographers new reference equations in the obstetrical practice.
References
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