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Fetal Biometry in the Population of Southern Punjab, Pakistan.

Byline: Durr-e-Sabih and Muhammad Kashif Rahim

Abstract

Background: Ultrasound estimation of fetal parameters is one of the most important examinations. Hadlock and other foreign fetal biometry curves are used in Pakistan as there are no national normative data derived from local population. Objectives: To construct local reference charts and equations for fetal biometric measurements and amniotic fluid index (AFI) using a large sample of fetuses examined at 14 - 40 weeks in the population of southern Punjab.

Study design, settings and duration: The prospective, cross sectional study conducted at the Multan Institute of Nuclear Medicine and Radiotherapy (MINAR) over a one year period from December 2010 to November 2011.

Subjects and Methods: A total of 566 randomly selected pregnant females, who fulfilled the inclusion criteria were included in the study after informed consent. Each woman was scanned once only, between 14 and 40 weeks of gestation, and her fetal measurements were also recorded simultaneously. Regression models were used to estimate the mean and standard deviation at each week of gestation. Fetal biometric parameters of local population were compared with those of Hadlock and Singaporean Asian population. Results were statistically analyzed and also presented graphically across the different gestational ages to allow visual comparison.

Results: Biometric measurements were obtained for 566 fetuses. There was no significant difference between fetal parameters of local population and those of Hadlock. When compared with those of the Singaporean Asian population, femur length and head circumference were larger while biparietal diameter was smaller in our population.

Conclusion: We have constructed local reference centiles for fetal measurements and equations for dating of pregnancy for Southern Punjab. The biometric measurements are slightly different from Singaporean Asian population, but there was no statistically significant difference between these measurements and those by Hadlock.

Key words: Fetal biometry, southern punjab, biparietal diameter, head circumference, femur length, abdominal circumference, amniotic fluid index.

Introduction

Fetal biometry is the methodology for measuring various aspects of fetal anatomy and growth.1 Fetal growth is the time dependent changes in the fetal body dimensions throughout the pregnancy. There is a rapid growth rate of different fetal biometric parameters, especially in first and second trimesters of gestation and this change of biometric parameters should be evaluated and correlated with normal value at that particular age.2

Before the development of ultrasound, fetal dimensions were measured using radiological techniques. Introduction of ultrasound made it safer and easier to measure soft tissue structures and bones of the fetus with more reliability and quicker when compared with x-rays. Donald and Brown were the first users of ultrasound for measuring the fetal biparietal diameter.3 Later others defined the basic methodology and fetal cephalometry.4

Over the last many decades, there has been a tremendous progress in the use of diagnostic ultrasound in obstetrical management due to its non-ionizing and non-invasive nature and its cost effectiveness resulting in wider acceptability.

Almost all ultrasound machines have standard charts of fetal biometry representing Western population though fetal biometric parameters vary with ethnic, social as well as nutritional status of population. A significant difference has been reported in the growth of Indian vs. non-Indian (Chinese and Malay) fetuses.5 Similarly, other investigators have also reported that the growth charts and femur length were different from in their local population.6,7 An Iranian researcher reported that fetal biometry in Iranian population a difference in femur length of local population with that from Hadlock et al used as a standard. The mean difference was 3.4 mm seen from 14 to 22 weeks and 5 mm afterwards.8

Similarly investigators from Cameron, Oman and Israel also reported that fetal biometric profile of their respective population was unique and different from other populations.9-11 Therefore, it is possible that fetal biometric charts for one population may under or over estimate the fetal age for another population and this may lead to erratic gestational age calculations, and wrong diagnosis of large-for-gestational age fetus or intrauterine growth restriction. Thus it is recommended that every country should construct and use biometric charts of its own population.

There is an effect of altitude on fetal weight and studies have shown a reduction of 102 grams - 145 grams in birth weight when altitude was increased up to 1000 meters, while keeping all other variables controlled.12-16

Zaidi et al from Pakistan established fetal measurements in population of Karachi, predominantly Urdu-speaking "Mohajirs" 17 but authors were unable to find any study on fetal biometry in the southern Punjab region, therefore this study was undertaken to calculate fetal biometry for this population.

Subjects and Methods

This prospective, cross sectional study was conducted at the Multan Institute of Nuclear Medicine and Radiotherapy (MINAR) over one year. The institute mainly caters for the population of southern Punjab. The study was conducted after approval by the ethics committee of MINAR. All measurements were performed by two trained ultrasound operators. The inclusion criteria were women from the Southern Punjab who had regular menstrual cycles of 26-33 days for at least three cycles before conception and who were certain of the date of the last menstrual period (LMP). The exclusion criteria were: (1) Multiple pregnancies (2) Uncertainty of LMP (3) Latent/overt diabetes mellitus (4) Pregnancy induced or chronic hypertension (5) History of maternal smoking (6) Rhesus (Rh) incompatibility (7) Fetal anomaly (or anomalies) detected on ultrasonography (8) Transverse lie after 34 weeks of gestation and (9) Any chronic maternal illness like tuberculosis, chronic liver disease or malignancy.

Mothers who gave an informed consent and who fulfilled the inclusion criteria were examined ultrasonographically and their Bi-Parietal Diameter (BPD), Femur Length (FL), Head Circumference (HC), Abdominal Circumference (AC) were measured in mm while Amniotic Fluid Index (AFI) was measured in cm. Only one ultrasound examination of each subject was performed by one of two operators. Real time transabdominal ultrasound was performed using Toshiba(r) Xario(r) SSA-660A and Toshiba(r) Nemio(r) SSA-550A ultrasound machines equipped with 3.5-MHz curvilinear abdominal probe.

Bi-parietal diameter and head circumference were measured in the axial plane having smooth and bilaterally symmetrical calvaria and where the midline echo in the anterior third was broken by the cavum septi pellucidi with symmetrical appearance of both thalami. Bi-parietal diameter was measured from the outer aspect of the proximal calvarial wall to the inner aspect of the distal calvarial wall. Fetal HC was measured over the outer to outer edges of calvaria by a computer-generated ellipse. Femur length measurement was done in a plane where fetal femoral diaphysis was seen parallel to transducer. In the last trimester, FL was measured more carefully not to include the epiphysis in the measurement. AC measurement was performed on a transverse fetal abdominal plane at the level where stomach bubble, fetal spine, umbilical vein (intra-hepatic portion) and the descending aorta are seen in the same plane. AFI was calculated by adding the vertical depth of the maximal pocket in each quadrant of the uterus.

Only those fetuses were included in the statistical analysis for which all measurements were available. For gestational age, fractions of weeks were computed to the nearest week, with fractions of <4 days being assigned to the lower week and equal or more than 4 days being assigned to the higher weeks.

Statistical analysis was done using the statistical software, Minitab(r) (Minitab Inc. USA). Anderson-Darling test was used to assess the normality of the measurements for each week of gestation and the Least-square regression model was used for the mean by fitting polynomial equation. A quadratic polynomial model was found the best for fitness to mean (y = a + b x GA + c x GA2).

Four fetal parameters (i.e, BPD, FL, HC and AC) of the local population were compared with those measured for Asian population of Singapore (visual comparison using graphs). The results were also compared with Western population (measured by Hadlock et al.) using Minitab(r) software (Minitab Inc. USA). Mean difference between population's parameters at each week of gestation was analyzed by means of independent two-sample t-test, as the data showed normal distribution. Comparison between two populations was done at each week of gestation separately for all four parameters. A p-value of <0.05 was taken as statistically significant.

Results

Biometric measurements were obtained for 566 fetuses. The mean maternal age was 26.63 +- 4.42 years. Polynomial regression model was fitted independently to the means of gestational age for each biometric measurement, using Minitab (Figure-1). The goodness of fit was assessed by their correlation coefficient (R2) and it was evident that quadratic polynomial model was the best model for fitness to mean. The following formula were obtained from the regression models and coefficients of correlation:

(GA = gestational age in weeks, calculated from last menstrual period; BPD = biparietal diameter, FL = femur length, HC = head circumference and AC = abdominal circumference; all measurements in mm):

BPD = - 37.07 + 5.157 GA - 0.04709 GA2 (R2 = 97.5)

FL = - 37.46 + 4.185 GA - 0.03437 GA2 (R2 = 97.9)

HC = - 150.1 + 20.20 GA - 0.1942 GA2 (R2 = 98.2)

AC = - 102.2 + 13.86 GA - 0.06197 GA2 (R2 = 97.9)

Anderson-Darling test was used to confirm the normality of the measurements for each week of gestation. The results showed that most of our data was normally distributed.

Centiles were calculated for each measurements using the formula: centile = mean + K x SD (K = +-1.88 for 3rd and 97th centiles). Mean, 3rd and 97th percentiles were plotted and comparison of the present study measurements was done with the corresponding measurements from a published study on Asian population by Lai FM et al.18 (Figure-2). The graphs showed that fetal biometrical parameters of our local population were slightly different from those reported by Lai et al.18 Our Femur length and head circumference values were slightly larger than those of Singaporean Asian population.

Though our results appeared slightly different from those of Hadlock et al. (Table-1 and Figure-3) but statistically no difference was found in all measurements.

Table 1: Comparison of fetal parameters of local population with those of Hadlock et al.

###Biparietal Diameter (mm)

Gestational Age (weeks)

###Our Study###Hadlock et al.

14###27###25

15###31###29

16###32###32

17###38###36

18###41###39

19###44###43

20###47###46

21###51###50

22###53###53

23###56###56

24###60###59

25###62###62

26###65###65

27###68###68

28###70###71

29###72###73

30###76###76

31###78###78

32###80###81

33###82###83

34###84###85

35###86###87

36###88###89

37###90###90

38###91###92

39###91###93

40###92###94

###Femur length (mm)

Gestational Age (weeks)

###Our Study###Hadlock et al.

14###14###15

15###18###18

16###20###21

17###24###24

18###27###27

19###30###30

20###33###33

21###35###36

22###38###39

23###41###42

24###43###44

25###46###47

26###49###49

27###50###52

28###52###54

29###55###56

30###56###58

31###59###61

32###61###63

33###64###65

34###66###66

35###68###68

36###69###70

37###71###72

38###71###73

39###73###75

40###75###76

###Head circumference (mm)

Gestational Age (weeks)

###Our Study###Hadlock et al.

14###98

15###116###114

16###126###122

17###141###134

18###152###148

19###165###160

20###175###177

21###188###182

22###198###193

23###211###208

24###222###221

25###232###239

26###245###241

27###256###256

28###262###271

29###272###273

30###284###277

31###293###281

32###300###292

33###306###302

34###314###309

35###321###317

36###326###322

37###335###330

38###336###336

39###338###340

40###344###345

###Abdominal circumference (mm)

Gestational Age (weeks)

###Our Study###Hadlock et al.

14###79

15###96###99

16###107###105

17###117###114

18###128###128

19###139###136

20###152###155

21###162###158

22###168###169

23###184###187

24###193###197

25###205###214

26###220###221

27###224###231

28###237###247

29###246###250

30###258###252

31###270###266

32###277###272

33###289###289

34###301###298

35###311###305

36###318###312

37###327###330

38###335###339

39###338###345

40###351###349

Table 2: Amniotic fluid index (AFI) (mean +- SD) at various weeks of gestation.

Gestational###AFI###+-SD###Gestational###AFI###+-SD

Age (Weeks)###(cm)###Age (Weeks)###(cm)

###mean###mean

14###11.1###+-1.2###28###15.7###+-3.3

15###12.5###+-1.0###29###14.7###+-2.6

16###11.6###+-2.1###30###14.5###+-3.4

17###13.3###+-2.2###31###15.0###+-3.2

18###13.2###+-1.2###32###14.8###+-2.7

19###14.7###+-1.5###33###15.5###+-3.3

20###14.3###+-2.5###34###14.4###+-5.0

21###15.2###+-2.1###35###14.8###+-3.9

22###15.9###+-1.9###36###13.4###+-3.2

23###16.2###+-2.7###37###14.8###+-4.9

24###16.1###+-2.8###38###14.1###+-4.5

25###17.0###+-3.3###39###14.8###+-3.3

26###15.8###+-3.2###40###14.3###+-2.9

27###16.5###+-2.2

Results of amniotic fluid index (AFI) was calculated in all subjects and were stratified by week of gestations (Table-2). The mean amniotic fluid index (AFI) was 14.982 cm (SD = 3.201cm). The graphical form of mean AFI for various weeks of gestation is shown in Figure-4.

Discussion

The present study on fetal biometry between 14 and 40 weeks of gestation was calculated for the population of Southern Punjab and the results showed that our femur length and head circumference values were slightly larger than those of Singaporean Asian population while statistically no difference was found in all measurements when compared with those of Hadlock et al.

Our sample size of 566 was not as big as in some other publications 7,19,20 but Leung et al. described that the sample size in hundreds is enough for data analysis.21 Increasing the sample further (1000 or in some studies even 2000 cases) increases the probability of including data from abnormal population subgroups.22

Fetal biometric measurements can be used to generate conversion tables for fetal gestational age 23 and this was also done in the present study. We compared the fetal parameters of our population with with Asian population in Singapore 18 using different gestational ages. The BPD measurements of our population and Singapore population were similar, although the Singapore population had a slightly larger BPD in the 3rd trimester of pregnancy (Figure-2). Femur Length was larger in our population when compared with the Singaporean population (Figure-2). Similarly Head Circumference was also larger in our population (Figure-2). However there was no difference in abdominal circumference of both populations (Figure-2).

We also compared the fetal parameters of local population with those of Hadlock et al. as database of Hadlock is used as the standard reference which is incorporated in most ultrasound machines for fetal age estimation. There was no significant difference between our parameters and Hadlock reference range. Mean amniotic fluid index of our population was 11 cm at 14 weeks, which gradually increased up to 17 cm at 25 weeks of gestation and then reduced gradually to 14 cm at 40 weeks of gestation. This was similar to the pattern described by other worker however their maximum median was 14 cm.24 Information about local normal range of AFI can immediately help to differentiate extremes of range (greater or less + 2SD) and deciding for frequent monitoring of patients who lie beyond this range. The present study not only describes the norms of local data but also validated the classical data (Hadlock) that is used by the local population.

Global data from pooled values has recently been published under the INTERGROWTH-21st project.25 It has included populations of 8 countries and is promoted as a generic standard of fetal growth. Local values can always be compared with classical Hadlock or the most recent INTERGROWTH-21st study.

Ultrasonologists must be aware that variation in different fetal parameters occur due to differences in the race, ethnicity, genetics and anatomical growth and are not necessarily due to abnormality. The presence of these charts makes it easier to discuss minor variation in fetal parameters within local population.

Acknowledgement

This project was partially funded by Pakistan Medical Research Council (PMRC) under grant no. 4-22-10/10/RDC/MINAR. We are thankful to the obstetricians of Nishtar Hospital Multan and Fatima Jinnah Hospital Multan for referring patients.

Conflict of interest: None declared.

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Publication:Pakistan Journal of Medical Research
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Date:Mar 31, 2017
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