ULTRASONOGRAPHIC COMPARATIVE STUDY OF BIPARIETAL DIAMETER IN FETUSES OF NORTH INDIAN WOMEN

A. K. Singh, Badal Singh

Department of Anatomy, Motilal Nehru Medical College, Allahabad

ABSTRACT This cross sectional study was conducted to evaluate accuracy of commonly used ultrasound dating

formulae in specified North Indian population. BPD of fetuses of 841 singleton pregnant females was ultrasonographically measured for subsequent statistical analysis. The mean BPD for each GA (derived from LMP) of this study was statistically compared with mean BPD from that of published dr~ta by Hadlock et al., Shepard & Filly, Sabbagha & Hughey and Kurtz et al.. Square Regression Equation was selected to prepare a population specific BPD table for every respective GA. The results show that North Indian fetuses are smaller than European fetuses even before 3rd trimester and gestational ages derived from sonographic Western reference equations are underestimated in this population, hence IUGR is diagnosed frequently; suggesting the need of population specific charts. It is generally thought that the small size of Indian neonates at birth is attributable to small maternal size, an inadequate nutrient supply during late pregnancy, or both; but that early fetal growth, when nutrient requirements are very small and there are no constraints on space for growth, is similar to that of other populations. This suggests that any intervention designed to ensure appropriate fetal growth in North Indian populations would need to start pre-conceptionally or during early pregnancy.

KEYWORDS: Ultrasound dating, BPD, North Indian population, IUGR.

INTRODUCTION: Size and body proportions at birth predict

short and long-term outcomes. The main determinant of perinatal mortality is low birth weight. As low birth weight can be caused by preterm delivery and/or IUGR, accurate assessment of fetal growth is a principal aim in antenatal care. Several development indicators e.g. BPD, HC, FL are used to predict the GA''2

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, when compared with standard charts, derived in white populations of developed countries, preloaded into ultrasound machines for ready references.

Birth size, however, is a crude summary measure of fetal growth, and two neonates of identical birth weight may have followed different growth trajectories5

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• For example, mean full-term birth weight of Indian neonates is 2.6-2.9 kg compared with 3.5-3.7 kg for white populations. But they are not proportionately smaller in all body measurements. Thus, use of charts derived from a

Correspondence Dr. Badal Singh Deptt. of Anatomy, Motilal Nehrn Medical College, Allahabad Mob.: 9415464436 email: drbadall999@gmail.com

J. Anat. Soc. India 61(2) 166-171 (2012) 166

different population may lead to errors in diagnosis of GA and over-diagnosis of IUGR.

The objectives of this study are to develop tables for ultrasound dating of pregnancy based on BPD, to derive reference curves for normal fetal growth based on biparietal diameter from 12 weeks of gestational age onwards and to compare these findings with previously published reference curves.

MATERIALS AND ,...,,ETHODS: A total of 841 singleton normal pregnant

females from specified North Indian population were enrolled after they met inclusion criteria. All fetal BPD measurements were taken by experienced doctors using Siemens G-50 and GE Logiq 400 pro ultrasound machines equipped with 3.5 MHz curvilinear trans-abdominal probe.

The transducer was as positioned so that the plane of section intersects the cavum septum pellucidi (CV), third ventricle and thalamus (TH); and cerebellar hemispheres were not visible in the scanning plane7

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• From the outer edge of the proximal to the inner edge ofthe distal skull margin in a transverse plane was measured as shown by arrow in following picture.

· · A K. Singh Badal Singh Ultrasonograph1c Comparative............................................. · •

STATISTICAL ANALYSIS We used only GA derived from LMP dates

rather than sonographic gestation because the latter assumes nearly identical growth in all fetuses and simply translates a measure of size into a gestational age using reference data.

In this study, collected data of measurement of the BPD was grouped by GA and mean BPD and SO were calculated.

According to central limit theorem we calculated a range of (mean)+/-(txSE) to locate the reference values of BPD.

To draw more informative conclusions from this study, comparison and analysis of variation between 2 groups were performed by using Unpaired t-test which was done by calculating differences of means (X1-X2), estimated standard error (SE), degree of freedom (df) and t-value (teal). Values of P for each companion were derived from "t table" with their respective degree of freedom and differences were accepted as statistically significant at a P- value <0.05. The method, we used to establish the relationship between two variables ie. BPD and GA and to generate nomogram of this study, is Regression correlational technique.

Differences in the performance of BPD prediction of GA between our nomogram and previously published nomograms of Hadlock et al, Kurtz et al and Shepard & Filly were compared.

OBSERVATION: For the purpose of statistical study, adjustments have been made to get the GA in complete figures as: "1 Owks 4days to 11 wks 3days" = 11 wks BPD measurements were tabulated against corresponding menstrual age (GA) and mean BPD for every completed GA; and Standard-Deviation (SO)

J. Anat. Soc. India 61(2) 166-171 (2012) 167

was calculated for every mean BPD on MS-Excel sheet with the help offormula:

SD= l:(X- X)2

n Where X = any BPD value

X = mean of BPD n = sample size or

According to the central limit theorem, 95% of sample means present within the range of:

"(mean) +/- ( t x SEest. )n.

Where SEest =Estimated Standard Error= SDlv'n

Statistic t (Student's-t) can be derived from t-tables or calculated with the formula: (table I)

t = X - J.l I SEest where X = sample mean

J.l = population mean

As the rule stated above, if population from which these samples were drawn, is same or comparable, 95% of these BPD means will be under this range, which can be appreciated simply by inspection of table II. Mean BPD measurements for each gestational age of this study was compared with mean BPD from that of Hadlock eta(/ which is most commonly used table for BPD by sonologists in Allahabad. No appreciable difference could be found up to mid 2nd trimester, but after that and especially in 3rd trimester, there was statistically significant [p<0.05] reduction in estimated values. To draw more informative conclusions from this study, comparison and analysis of variation between 2 groups were performed by using Unpaired t-test which was done by calculating differences of means (X1-X2), estimated standard error (SE). degree of freedom (df) and t-value (teal); and statistically significant [p<0.05] reduction in estimated BPD values of our study in comparison with Western reference values were observed, especially in later half of pregnancy.

Ultrasonographic Comparative ............................................. A. K. Singh, Badal Singh

From the findings of Unpaired t-test, table II and comparison of frequency polygons shown in chart 1,

we can observe that fetal BPD vary from population to

population prompting the need of specific nomograms offetal measurements.

Biomedical research often seeks to establish if there is a relationship between two variables; eg. BPD and GA9

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'11 The methods used to do this are

correlational techniques, which can be of two basic kinds: 1. Correlation: used to establish and quantify the strength and direction of the relationship between

two variables. It can be presented graphically in the form of scattergram. (Chart:2) 2. Regression: used to express the functional

relationship between two variables, so that the value of one variable can be predicted from the knowledge

of the other. In its most basic form, simple linear regression, the straight regression line is actually the

same "line of best fit" to the scattergram. Likewise we can observe Square, Cubic and Quadratic Regression

equations simply with the help of scattergram (Chart:3), in which regression lines can be

represented by respective equations, and R2 =

Coefficient of determination (expresses proportion of variance in Y for variance in X). Now a day, SPSS

software is frequently being used with fair degree of

accuracy and simplicity in this type of complex statistical analysis, using correct intercepts and constants as: 1. Linear Regression Analysis:

BPD=-5.82484 + 2.59826 x GA R2 = 94.9437 GA = 3.40336 + 0.366833 x BPD R2 = 94.7192

2. Square Regression Analysis:

BPD = -28.8345 + 4.53833 x GA 0.0368507 x GA2 R2 =95.8874% GA = 7.98351 + 0.18994 x BPD + 0.00147292 x BPD2 R2 =95.3003%

3. Cubic Regression Analysis:

BPD =- 20.7908 + 3.479 x GA + 0.00646396 x GA2-0.000555728 X GA3 R2 = 95.896% GA = 10.2933 + 0.0436639 x BPD + 0.00421134 x BPD20.0000156145 x BPD3 R2 = 95.3289%

Taking the inference from all the observations, Square Regression Equation was selected to prepare a BPD (mm) table for every

respective GA (wks), which is presented in table Ill, in comparison with other existing nomograms.

Table I : GA, Sample Size, Mean BPD, SD, SEest and t-Values of study

GA BPD (SD) SE t·Value' for GA BPD (SD) SE t·Value' for

(sample) level of (sample) level of

Slgnlllcanct Slgnlllcanct

P<O.OS P<O.OS

10 (2) 20.45(0.6) 0.629 12.706 27 (32) 66.96(5 .7) 1.028 2.04

11 (2) 20.02(0.1) 0.141 12.706 28 (26) 71.32(3.5) 0.692 2.06

12 {12) 20.19(1.9) 0.561 2.201 29 (28) 71.99(3.6) 0.702 2.052

13 (20) 22.95(3.1) 0.713 2.093 30 {18) 73.79(3.8) 0.932 2.11

14 (21) 26.81(3.3) 0.743 2.086 31 (19) 75.22(6.7) 1.587 2.101

15 {31) 31.41(3.6) 0.662 2.042 32 (22) 79.21(3.9) 0.846 2.08

16 (25) 32.41(4.7) 0.962 2.064 33 (28) 79.26(4.5) 0.858 2.052

17 {36) 36.92(4.2) 0.71 2.03 34 (26) 82.84(3.7) 0.742 2.06

18 (34) 41.31(3.3) 0.576 2.035 35 (46) 83.89(6.0) 0.9 2.01

19 (33) 44.39(3.9) 0.692 2.038 36 (55) 87.15(3.3) 0.451 2.005

20 (41) 46.44(6.3) 0.989 2.021 37 (43) 88.58(4.4) 0.678 2.017

21 (27) 49.98(3.4) 0.673 2.056 38 (32) 90.81(4.7) 0.84 2.04

22 (27) 52.24(5.2) 1.018 2.056 39 (17) 90.3(4.1) 1.031 2.12

23 (26) 53.84(4.5) 0.903 2.06 40 (17) 92.64(4.1) 1.036 2.12

24 (29) 58.07(6.5) 1.232 2.048 41 (8) 94.28(4.4) 1.656 2.365

25 (35) 60.47(3.4) 0.582 2.033 42 (2) 96.45(1.8) 1.768 12.706

26 (21) 66.12(6.2) 1.376 2.086

* t-value was derived from Student's t-table with the help of df. Degree of freedom (df) = n-1

168

Ultrasonographic Comparative .•.•..•.•....•.•..•....•....•.•......•.••.•. .A. K. Singh, Badal Singh

Table II : Correlation of Predicted Menstrual Age based on Biparietal Diameter

Correlation of Predicted Menstrual Age based upon Biparietal Diameter

95% Pop. MeanBPD(X) Means lie between

GA(Wks) X- X+ Hadlock Shepard Kurtz Sabbagba This t*(SE) t*(SE) etal &FIIy etal. " Study

1982 1982 Hughey 13 21.46 24.44 22.95 14 25.26 28.36 27 28 27 28 26B1 15 30.06 32.76 30 31 31 32 31.41 16 30.42 34.40 33 34 34 36 32.41 17 35.48 38.36 37 37 38 39 36.92 18 40.14 42.48 40 40 41 42 41.31 19 42.98 45.80 43 43 45 45 44.39 20 44.44 48.44 46 46 48 48 46.44 21 48.60 51.36 50 49 51 51 49.98 22 50.15 54.33 53 52 54 54 52.24 23 51.98 55.70 56 55 57 58 53.84 24 55.55 60.59 58 57 60 61 58.07 25 59.29 61.65 61 60 63 64 60.47 26 63.25 68.99 64 63 66 67 66.12 27 64.86 69.06 67 65 69 70 66.96 28 69.90 72.74 70 68 71 72 71.32 29 70.55 73.43 72 71 74 75 71.99 30 71.82 75.76 75 73 76 78 73.79 31 71.89 78.55 77 76 79 80 75.22 32 77.45 80.97 79 78 81 82 79.21 33 77.50 81.02 82 80 83 85 79.26 34 81.31 84.37 84 83 85 87 82.84 35 82.08 85.70 86 85 87 88 83.89 36 86.25 88.05 88 88 89 90 87.15 37 87.21 89.95 90 90 91 92 88.58 38 89.10 92.52 91 92 92 93 90.81 39 88.11 92.49 93 95 94 94 90.3 40 90.44 94.84 95 97 95 95 92.64 41 90.36 98.20 94.28

100

120 90

,...100 I ! 80

I 60

- Thl~ St J:fv: M.•oll' BPO I 40

... (.., 60

-!';(jk'o(a.ctlltf'l~!

20

0

'" 0 10 20 w 40 so GA(wks)

10

lS

Chart 2: Scattergram showing positive correlation between GA and BPD in this study

Chart 1: Comparison of BPD means (mm) of study & existing nomograms J. Anat. Soc. India 61(2) 166·171 (2012) 169

Ultrasonographic Comparative A · ............................................. . K. Smgh, Badal Singh

----------·-- ----- -~------ ··--------- -· 150 linear re&ression equatl~~ ---- --------- - ---·------

...... 1100 .... c t so

120

100 '

0 '

0 10 40

• BPD(-)

- Llnear(BPD(••JJ

so

Chart 3: Scattergrams showing linear, Square, Cubic & Quadratic regression equations between GA & BPD (without use of intercept)

DISCUSSION &CONCLUSION: Accurate assessment of GA by sonography can be of great importance in management decisions during pregnancy. Even in women with reliable dates, errors in gestation calculation can arise; therefore, ultrasound predictions from a regression line should be more accurate 12

• This study presents sonographically derived measurements of fetal BPD growth from a North Indian population and compared

them against the Western population. Fetal BPD was found significantly smaller than the western references after 22 wks' gestation and variation increased as pregnancy progressed. Hence, GA tends to be underestimated and IUGR is

diagnosed frequently (~30%)6· 13 • Our results of comparison with Pakistani nomogram

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along with Western tables8'10 support the idea that

there is an ethnic and racial variation in different populations for biometric measurements and

J. Anat. Soc. India 61(2)166-171 (2012) 170

GA Derived Hadlock Kurtz et al. Shepard & (wks) BPD of et al. (Composite) Filly 1982

this 1982 study (mm)

14 27.48 27 27 28 15 30.95 30 31 31 16 34.35 33 34 34 17 37.67 37 38 37 18 40.92 40 41 40 19 44.09 43 45 43 20 47.19 46 48 46 21 50.22 50 51 49 22 53.17 53 54 52 23 56.05 56 57 55 24 58.86 58 60 57 25 61.59 61 63 60 26 64.25 64 66 63 27 66.84 67 69 65 28 69.35 70 71 68 29 71.79 72 74 71 30 74.15 75 76 73 31 76.44 77 79 76 32 78.66 79 81 78 33 80.80 82 83 80 34 82.87 84 85 83 35 84.86 86 87 85 36 86.79 88 89 88 37 88.64 90 91 90 38 90.41 91 92 92

Table Ill : Comparison between derived BPD (mm) of this study with existing nomograms

emphasize the need for a locally generated fetal growth reference. It is generally thought that small size of Indian neonates at birth is attributable to small maternal size,.. an inadequate nutrient supply during late pregnancy, or both; but early fetal growth, when nutrient requirements are very small and there are no constraints on space for growth, it should be similar to that of other populations6

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• This suggests that any interventions to optimize fetal growth in North Indian mixed populations would need to occur pre­conceptionally or during early pregnancy also along with mid or fate pregnancy. With the help of following Square regression equation, this study provides population specific reference chart for ultrasound dating of pregnancy based on BPD. BPD = -28.8345 + 4.53833 x GA 0.0368507 x GA2 or GA = 7.98351 + 0.18994 x BPD + 0.00147292 x

Ultrasonographic Comparative ............................................. A. K. Singh, Badal Singh

BPD2 GA derived from a reliable, population specific growth curves can improve obstetric management. Our findings need to be replicated in other Indian populations with data collected for other fetal biometric parameters.

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