1.      Introduction

The assessment of fetal weight is an important indicator for the fetal nutritional state and one of the factors affecting critical obstetrics decisions [1,2]. Over the last decades estimation of the fetal weight was based on 2D ultrasound formulae which had the disadvantage of being inaccurate as shown in pervious systematic reviews [3]and also failed to predict neonatal adipose tissue status which is more affected by nutritional status [4]. Significant improvement of the measurements was achieved after incorporating measurements of the thigh volume using 3D ultrasound [5]. Fractional limb volume is a fetal soft tissue parameter that is based on 50% of the long bone diaphysis length to avoid the falsies obtained from difficult volume acquisition near the end of long bones [6]. Further improvement in accuracy was recorded following the use of VOCAL technique which can be more precise in obtaining volume from regular shaped objects [7]. However, 3D-ultrasound still requires time and effort in reconstructing the image and is affected by the angle used and the experience of the sonographer which affects its reproducibility. To overcome these defects, long bone automated detection system, five-dimensional 5D Long Bone (5D LB) was introduced with an automated system that allow the volume measurement to be completed in just a few seconds and eliminate operators’ variability which makes it more useful in clinical practice [8]. Also, the fact that ethnic and racial variation exists in fetal biometry [2], mandate testing the hypothesis that 5D or 3D ultrasonography measurement of fetal thigh volume may be more accurate in prediction of fetal weight in comparison to the conventional two-dimensionalHadlock formula in this study population

2.      Patients and Methods 

This study is a prospective study conducted at Zagazig University Hospitals, and Agial Fertility Center. Accordingly, throughout the period between June and December 2016, 50 pregnant women with singleton pregnancy at 37 to 41 weeks of gestation, who were admitted for planned delivery within 48 hours either by induction of labor or elective caesarean section, were enrolled. Gestation age was calculated from the first day of the Last Normal Menstrual Period (LMP) provided it is sure and reliable (regular cycles for the preceding three months with no history of hormonal contraception or recent termination of pregnancy). Otherwise gestation age was calculated from early first -trimester ultrasound with crown rump measurement. Patients with fetal anomalies, abnormal amount of liquor and factors influencing proper measurements as pelvic lesions were excluded from the study. Demographic data were recorded and all patients underwent a formal 2D ultrasound scan by the same examiner to calculate the expected fetal weight by using the Hadlock IV model, which incorporates Biparietal Diameter (BPD), head circumference, Abdominal Circumference (AC) and femoral diaphysis length (FL) [9]. 3D ultrasonography was used by another examiner blinded to the previous measurements for thigh volume measurement according to the principle described by Benini et al. [7]. “The conventional plane for measurement of femur length was first identified for orientation of the thigh then the plane was rotated to put the femur accurately in a horizontal position. A stepwise measurement using the Virtual Organ Computer-Aided Analysis (VOCAL) technique were performed as follows: The data set containing the fetal thigh was initially displayed on the screen in three orthogonal planes, the sagittal view of the femur were displayed in Plane A and this image were rotated so that the orientation of the thigh and whole diaphysis coincides with the y-axis. Two demarcating arrows were positioned at each end of the diaphysis to define the limits of the thigh to be included in the volume calculation. Volume estimates were computed utilizing the VOCAL program with a manual trace at 30 of rotation. At the end of the 180 rotation, the built in software was used to calculate the volume automatically” Birth Weight (BW) were calculated through the following formula BW = 1025.383 + 12.775 × Thigh volume. Biometric measurements were taken as the average of 2 readings[10, 11]. The machine used for examination was Voluson E6 BT12 with a volumetric abdominal probe RAB 6D-4D curved Array (General Electric Medical Systems, AUSTRIA). Subsequently, the long bone length was measured by another analyzer using the 5D LB with the following procedures described by Hurr et al. [8]. “The volume data used in the manual 3D-ultrasound measurement were displayed in an offline multiplanar mode, and the 5D LB set key was pressed on the system, wherein the system automatically analyzed the 3D volume data, reconstructed the 3D image of the long bones, and displayed the measured lengths of the long bones on the screen”. After delivery all neonates’ weights were obtained using the same digital weight scale immediately after birth and recorded in the hospital files.

4.1. Ethical Consideration

Institutional Review Board (IRB) approval: The protocol was discussed by the ethical scientific committee for approving the study and informed consent was obtained before participation. Acceptance of local institutional committee and the ethical committee of the faculty of medicine was obtained before commencing the trial and all participating women signed a written informed consent after proper explanation. 

2.2.  Sample size calculation 

The required sample size has been calculated using G*Power software version 1.1.7 (Germany). The primary outcome measure is the accuracy of 2D, 3D or 5D ultrasonography for estimating the actual weight of the newborn obtained immediately after delivery So, it was estimated that a total sample size of 50 patients on whom estimation of the birth weight was undertaken would achieve a power of 90% (type II error, 0.1) to detect a statistically significant difference between the overall accuracy of any two techniques for a median effect size (Cohen’s dz) of 0.5 using a two-sided paired t test with a confidence level of 95% (type I error, 0.05). This effect size has been chosen as it could be regarded as a clinically relevant difference to seek in this study. 

3.      Results               

50 women underwent the three modalities of ultrasound within 48 hours of delivery. The characteristics’ of the included patients are summarized in Table 1. Comparing the accuracy of 2D ultrasound to 3D ultrasound in the assessment of birth weight (Table 2) showed that 2D estimated fetal weight was significantly less accurate than 3D estimated fetal weight as measured by absolute birth weight estimation error and percent birth estimation error. On the other hand comparing the accuracy of 5D to 3D ultrasound showed a statistical significance in favor of the 5D but the difference was so small to impose a clinical significance in obstetric practice.

Signed error is the estimated weight by US minus the actual birth weight.Signed percentage error is the estimated weight by US minus the actual birth weight/actual birth weight × 100.Absolute error is the unsigned difference between the estimated weight by US and the actual birth weight.Absolute percentage error is the unsigned difference between the estimated weight by US and the actual birth weight/actual birth weight ×100.t, t statistic; df, degree of freedom.¶Paired Studentt-test. 

4.      Discussion 

The accurate prediction of birth weight is essential not only in macrosomic fetus to avoid unplanned birth injuries or operative deliveries but also in low birth weight growth restricted fetus to avoid perinatal asphyxia [12-14].Previous studies demonstrated up to 10%standard error for most of the commonly used 2D formulae for estimation of fetal weight specially at the birth weight extremities [5] It is debatable if this observation is attributed to inter-observer variability or to the lack of incorporation of soft tissue measurements in most of these formulae [2]. Subsequently improvements in the accuracy of BW estimation were achieved after incorporating measurement of fetal weight using 3D with earlier study showing absolute percentage errors of less than 6% [7].

 In the current study 2D EFW was significantly less accurate that 3D EFW as measured by absolute BW estimation error & absolute percentage BW estimation error. Also in this study, 3D U/S EFW was significantly more precise than 2DU/S EFW as determined by absolute BW estimation error & absolute percentage BW estimation error. These results agreed with the previous work of Schild et al 2000, Isobe 2004 and  Sriantiroj et al. 2009[15-17] who agreed that fractional ThV was the best predictor for actual birth weight and is superior to 2D U/S formulae which need head measurement which is usually inaccurate at term pregnancy especially if the fetal head is deeply impacted in the pelvis and also lacks the ability to assess the effect of fat distribution in the limbs, facts which further compromised fetal-weight estimation by 2D formulae . 

On the other hand, Lindell et al. 2009[18]reported no difference between 2D and 3D ultrasound in the estimation of fetal weight in a group of women with post term pregnancy ,a different cohort from our study population ,Also Bellini et al 2011 postulated that the previous superiority of 3D formulae over 2D might be attributed to phenotypic differences between different  patients used to create each of these formulae [7]Yang et al 2011 emphasized on the fact that ethnic and racial variations can significantly affects fetal biometry[2] which prompt careful interpretation of data obtained from different studies. 

Despite the obvious superiority of 3D ultrasound in estimation of fetal birth weight, the technique is still operator dependent and requires a learning curve for proper acquisition and manipulation of volume data [8]. In an effort to overcome this drawback, long bone automated detection system by 5D was introduced to create an operator independent, quick and efficient method for accurate estimation of fetal birth weight. In the current trial, this fully automated system revealed absolute birth weight estimation error of 0.95% which is comparable to the previous work of Hurr et al who reported an overall error rate of 5.4% in a larger sample[8]. 

In the current trial,3D assessment of fetal volume was done using the VOCAL technique with a 30^◦ rotation angle which was previously shown by Benini et al. 2011[7] to be significantly faster than multiplanner method (P < 0.001).A former trial reported that 3D volume data was acquired within 2 minutes and interpreted in 6 to 7 minutes [19].

 The points of strength in this study lies in its ability to complete the three modalities in all patients who were examined with the same examiner for each technique, all patients were delivered within 48 hours from the ultrasound scan detected to avoid falsies from longer intervals and birth weights were recorded by the same digital weight scale attended by an examiner to ensure accuracy. 

On other hand the authors recognize the fact that fetuses with abnormal growth were not assessed as the random selection resulted in a study population which was within normal range of birth weight.The implication of these findings on babies in the extremes of body weight might be a point of interest for future research.

Table 1: Characteristics of the study populationData are mean ± SD or number (%).

Table 2: Comparison of the accuracy of 5D US versus 3D US and 2D US versus 3D US for estimation of birth weightData are mean ± SD.

1.       Lee W, Deter R, Sangi-Haghpeykar H, Yeo L (2013) Romero R.Prospective validation of fetal weight estimation using fractional limb volume. Ultrasound ObstetGynecol41: 198-203.

2.       Yang F, Leung KY, Hou YW, Yuan Y, Tang MH (2011) Birth-weight prediction using three-dimensionalsonographic fractional thigh volume at term in a Chinese population. Ultrasound ObstetGynecol 38:425-433.

3.       Dudley NJ (2005) A systematic review of the ultrasound estimation of fetal weight. Ultrasound ObstetGynecol; 25: 80-89.

4.       Moyer-Mileur LJ, Slater H, Thomson JA, Mihalopoulos N, Byrne J, et al. (2009) Newbornadiposity measured by plethysmography is not predicted by late gestation two-dimensional ultrasound measures of fetal growth. J Nutr 139: 1772-1778.

5.       Schild RL (2007) Three-dimensional volumetry and fetal weight measurement. Ultrasound ObstetGynecol 30: 799-803.

6.       Lee W, Balasubramaniam M, Deter RL, Yeo L, Hassan SS, et al. (2009) New fetal weight estimation models using fractional limb volume. Ultrasound ObstetGynecol34:556-565.

7.       Beninni JR, Faro C, Marussi EF, Barini R, Peralta CFA (2010) Fetal thigh volumetry by three-dimensional ultrasound: comparison between multiplanar and VOCAL techniques. Ultrasound ObstetGynecol; 35: 417-425.

8.       Hur H, Kim YH, Cho HY, Park YW, Won HS, et al. (2015) Feasibility of three-dimensional reconstruction and automated measurement of fetal long bones using 5D Long Bone..ObstetGynecol Sci 58:268-276

9.       Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK (1985) Estimation of fetal weight with the use of head,body, and femur measurements-a prospective study. Am J ObstetGynecol 151: 333-337.

10.    Pitman E (1939) A note on normal correlation. Biometrika; 31:9-12

11.    Bland JM, Altman DG (2003) Applying the right statistics:analyses of measurements studies. Ultrasound ObstetGynecol 22: 85-93.

12.    Melamed N, Yogev Y, Meizner I, Mashiach R, Bardin R, et al. (2009) Sonographic fetal weight estimation: which model should be used? J Ultrasound Med 28: 617-629

13.    Orskou J, Kesmodel U, Henriksen TB, Secher NJ (2001)an increasing proportion of infants weigh more Than 4000 grams at birth. Acta ObstetGynecolScand80: 931-936.

14.    Ecker JL, Greenberg JA,Norwitz ER, Nadel AS, Repke JT (1997) Birth weight as a predictor of brachialplexus injury. ObstetGynecol 89: 643-647.

15.    Schild RL, Fimmers R, Hansmann M (2000) fetal weight estimationby three-dimensional ultrasound. Ultrasound ObstetGynecol 16: 445-452

16.    Isobe T (2004): Approach for estimating fetal body weight using two-dimensional ultrasound. J Maternal Fetal Neonatal Med 15: 225-231.

17.    Srisantiroj N, Chanprapaph P, Komoltri C (2009) Fractional thigh volume by three-dimensional ultrasonography for birth weight prediction. J Med Assoc Thai 92:1580-1585.

18.    Lindell G, Marsal K (2009) Sonographic fetal weight estimation in prolonged pregnancy:comparative study of two- and three-dimensional methods. Ultrasound ObstetGynecol 33: 295-300.

19.    Benacerraf BR, Shipp TD, Bromley B (2006) Three-dimensional US of the fetus: volume imaging. Radiology 238:988-996.