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New birth weight reference standards customised to birth order and sex of babies from South India

  • Velusamy Saravana Kumar1,
  • Lakshmanan Jeyaseelan1,
  • Tunny Sebastian1,
  • Annie Regi2,
  • Jiji Mathew2 and
  • Ruby Jose2Email author
BMC Pregnancy and Childbirth201313:38

DOI: 10.1186/1471-2393-13-38

Received: 11 May 2012

Accepted: 12 February 2013

Published: 14 February 2013

Abstract

Background

The foetal growth standards for Indian children which are available today suffer due to methodological problems. These are, for example, not adhering to the WHO recommendation to base gestational age on the number of completed weeks and secondly, not excluding mothers with risk factors. This study has addressed both the above issues and in addition provides birthweight reference ranges with regard to sex of the baby and maternal parity.

Methods

Data from the labour room register from 1996 to 2010 was obtained. A rotational sampling scheme was used i.e. the 12 months of the year were divided into 4 quadrants. All deliveries in January were considered to represent the first quadrant. Similarly all deliveries in April, July and October were considered to represent 2nd, 3rd and 4th quadrants. In each successive year different months were included in each quadrant. Only those mothers aged 20–39 years and delivered between 24 to 42 weeks gestational age were considered. Those mothers with obstetric risk factors were excluded. The reference standards were fitted using the Generalized Additive Models for Location Scale and Shape (GAMLSS) method for Box – Cox t distribution with cubic spline smoothing.

Results

There were 41,055 deliveries considered. When women with risk factors were excluded 19,501 deliveries could be included in the final analysis. The male babies of term firstborn were found to be 45 g heavier than female babies. The mean birthweights were 2934 g and 2889.5 g respectively. Similarly, among the preterm babies, the first born male babies weighed 152 g more than the female babies. The mean birthweights were 1996 g and 1844 g respectively.

In the case of later born babies, the term male babies weighed 116grams more than the females. The mean birth weights were 3085 grams and 2969 grams respectively. When considering later born preterm babies, the males outweighed the female babies by 111 grams. The mean birthweights were 2089 grams and 1978 grams respectively. There was a substantial agreement range from k=.883, (p<.01) to k=.943, (p<.01) between adjusted and unadjusted percentile classification for the subgroups of male and female babies and first born and later born ones.

Birth weight charts were adjusted for maternal height using regression methods. The birth weight charts for the first born and later born babies were regrouped into 4 categories, including male and female sexes of the babies. Reference ranges were acquired both for term and preterm babies.

With economic reforms, one expects improvement in birthweights. The mean (sd) birthweights of the year 1996 was 2846 (562) as compared to year 2010 (15 years later) which was 2907 (571). There was only a difference of 61 grams in the mean birthweights over one and a half decade.

Conclusion

New standards are presented from a large number of deliveries over 15 years, customised to the maternal height, from a south Indian tertiary hospital. Reference ranges are made available separately for first born or later born babies, for male and female sexes and for term and preterm babies.

Keywords

Reference Foetal growth Birth weight Gestational age Preterm Modelling Box–Cox t Cubic spline smoothing

Background

Investigators and researchers have used the reference standards which were developed globally due to lack of availability of population or country specific standards. A decline in the use of these standards has been reported [1] as regionally developed standards are made available [25]. Mohan et al. (1990) have reported growth curves for North Indian babies from a referral hospital [1]. However, these reference standards have limitations as they failed to exclude mothers and babies with risk factors and the hospital predominantly served mothers with low socioeconomic status. Most of the above studies have not adhered to the World Health Organization (WHO) recommendation to base gestational age on number of completed weeks. The latest report on birth weight standards for South Indian babies was published in 1996, which is nearly one and a half decades old and may no longer be pertinent to infants born in more recent years [2]. The country has gone through economic revolution in the last two decades, which has influenced all sections of society. Therefore, the birth weight of the new born babies is expected to increase. Kramer et al. have reported the validity of the calculation of gestational age from the last menstrual period and therefore the credibility of the birth weight standards which were published earlier [36]. The gestational age from 24 weeks to 29 weeks are expected to be small in numbers and therefore likely to have a skewed distribution in birth weight. Some studies have failed to smooth the standard curves using appropriate distribution such as log normal or Box – Cox t distribution with cubic spline smoothing [7, 8]. In this study we have overcome the above mentioned limitations by studying children born from 1996 to 2010 in a referral hospital which has a unique medical records system, with appropriate inclusion criteria, foetal sex and mother’s gravidity and gestation specific growth standards for Indian children.

Methods

The Christian Medical College and Hospital is a referral hospital in South India, which caters to 47,110 outpatients and 15,662 inpatients per year. The department of Obstetrics and Gynaecology on an average delivered 20 babies per day in 1996 and 40 babies per day in 2010. The data was obtained from the Labour room register which was maintained by the nursing personnel and supervised by the Head of Obstetrics department. This contains all the information about women who delivered in this institution. Permission was obtained from all the Obstetric departmental Unit Heads. This study was approved by the ethics committee of [IRB Min. No. 7109 dated 10.03.2010] Christian Medical College.

Sampling

The twelve months of the year were divided in to 4 quadrants. In the year 1996, all deliveries which took place in January were considered to represent the first quadrant. In the second quadrant, all deliveries in the month of April were considered. All deliveries in the months of July and October were considered to represent the deliveries in the third and fourth quadrants. In the second year of the study (1997) all deliveries which took place in February, May, August and November were taken to represent the first, second, third and fourth quarter of the year. In the third year of study (1998), all deliveries which took place in March, June, September and December were taken to represent the first, second, third and fourth quarter of the year. The above cycle was repeated for the next 3 years and continued until the year 2010 [9].

Inclusion and exclusion criteria

Mothers aged 20 to 39 years and deliveries of gestational age between 24 weeks to 42 weeks were considered. The mothers, who had hypertensive disorders, gestational diabetes, diabetes mellitus, heart disease, and twin pregnancies, were excluded from the analyses. The gestational age specific birth weights which were above +3SD or below –3SD values were excluded from the analyses. The maternal height was regrouped into tertiles with categories as < 151 cm, 151 – 158 cm and > 158 cm based on the maternal height distribution of the mothers. Linear regression was employed to get the estimate of birthweight in relation to the maternal height. It showed a significant increase in birthweight of 135 grams. These 135 grams were added to the birthweight for shorter women and for taller women 135 grams were subtracted. Women with normal height (151–158 cm) did not have any adjustments in birthweight. These corrections yielded us the birthweights adjusted for maternal height.

The modified birthweights were adjusted with gestational ages to produce the birthweight centiles for each of the four groups in the term as well as the preterm groups. (Male & Female first born, Male & Female later born).

Distribution and smoothing

The birth weights adjusted for maternal height for babies born from gestational age 24 weeks to 30 weeks had skewed distribution to the right side. However, as the number of deliveries increased in the subsequent gestational weeks, the birth weight distribution followed normal distribution. In the modelling we have assumed Box-Cox t distribution in order to get over this skewness. Cubic Spline smoothing has been applied using the Generalized Additive Models for Location Scale and Shape (GAMLSS) method using R software (Stasinopoulos and Rigby 2007) [10]. The GAMLSS models are semi parametric regression type models. However, the response variable needs to follow parametric distribution. The 3rd, 10th, 25th, 50th, 75th, 90th and 97th percentiles and mean and standard deviations were computed using R software. These centiles for birthweight adjusted for maternal height were done separately for firstborn males, females and later born males and females.

Data from the labour room register was entered using EPIINFO software. Completed weeks gestational age was considered for nomograms. The best estimate of gestation based on reliable menstrual history, early antenatal clinical examination and sonographic fetal biometry was used. Birth weights were measured to the nearest 50 g on a Braun electronic weighing scale within one hour of birth.

Results

In total, there were 41,055 deliveries considered. Of these, complete data were available for 25,090 deliveries. When women with risk factors were excluded (mild and severe PIH, Chronic hypertension, GDM, Pregestational diabetics, cardiac disease, twins, teenaged primigravidas and mothers more than 40 years of age), 19,501 deliveries could be included for analyses. Most of the women were in the age group 25 – 29 years 41.7% (8133). Close to this is the 20 to 24 years age group which constituted 40.1% (7817). Mothers more than 30 years constituted only 18.2% (3551).

The maximum number of mothers who delivered belonged to the Hindu religion, and formed 82% (15933). Muslims formed 11.1% (2164), and Christians constituted 6.9% (1343).

Most (91.8%) of the mothers were housewives. Professionals were a minority, 4.3% (844), and mothers trained for skilled work were only 1.4% (241). When considering education of the mothers, 28.6% (4961) were graduates or more highly educated. A small percent was illiterate 5.1% (895). Those who had primary education formed 6.2% (1085) and those with secondary education 13.6% (2367). Those who had high school and higher secondary education constituted 8099 (46.5%) of the mothers.

When sub grouped into 3, based on the heights of the mothers, 24%, 48% and 28% of the mothers fell into the 3 groups ,whose height was <151 cm; 151-158 cm and >158 cm respectively.

There were 2379 firstborn and 17,092 later born. There were 1236 (6.3%) male babies in the first born group and the first born females were 1143 (5.9%) in number. Among the later born babies, male babies were 8739 (44.9%) in number and females 8353 (42.9%).

Term and preterm first born babies

The male babies of term first born mothers were found to be 45 grams heavier than female babies. The mean birthweights were 2934 grams and 2889.5 grams respectively. Similarly, among the preterms, male babies weighed 152 grams more than the female babies. The mean birthweights were 1996 grams and 1844 grams respectively.

Term and preterm later born babies

In the case of later born babies, term male babies weighed 116 grams more than the females. The mean birth weights were 3085 grams and 2969 grams respectively. When considering preterm babies among the later born, the males outweighed the female babies by 111 grams. The mean birthweights were 2089 grams and 1978 grams respectively.

Table 1 shows the 3rd, 10th, 25th, 50th, 75th, 90th, 97th percentile, mean and SD birth weights for male and female babies of primigravidae. Figures 1 and 2 shows the smoothed percentile curves for male and female babies of first born separately. The growth curves for various percentiles are smooth and increasing steadily as gestational age increases.
Table 1

Smoothed percentiles for birth weight (grams) of first born male and female babies

First born male babies smoothed percentiles

First born female babies smoothed percentiles

GA

N

C3

C10

C25

C50

C75

C90

C97

Mean

SD

N

C3

C10

C25

C50

C75

C90

C97

Mean

SD

31

8

590

871

1131

1394

1637

1842

2035

1427

261

6

601

773

953

1159

1369

1562

1755

1146

310

32

13

744

1055

1340

1630

1899

2126

2340

1513

496

8

774

976

1187

1426

1670

1894

2119

1400

395

33

13

939

1262

1558

1861

2142

2381

2606

1820

529

10

961

1186

1421

1687

1958

2207

2456

1733

397

34

23

1164

1482

1777

2079

2362

2604

2833

2112

358

11

1157

1399

1649

1934

2223

2488

2754

2076

422

35

33

1379

1691

1981

2282

2566

2809

3039

2300

450

16

1366

1616

1874

2166

2464

2735

3007

1941

417

36

53

1591

1892

2176

2472

2752

2993

3222

2353

486

43

1598

1847

2104

2393

2688

2957

3225

2410

433

37

124

1806

2092

2365

2652

2925

3160

3385

2687

384

81

1821

2063

2311

2591

2875

3134

3392

2637

424

38

196

1982

2259

2524

2804

3072

3304

3525

2774

429

184

2004

2236

2474

2741

3012

3259

3505

2742

397

39

269

2152

2417

2673

2944

3204

3429

3645

2912

388

265

2142

2370

2604

2866

3132

3374

3615

2856

384

40

308

2287

2547

2798

3065

3321

3544

3758

3083

391

321

2250

2479

2714

2977

3244

3487

3729

2994

404

41

82

2366

2614

2855

3112

3359

3575

3781

3078

383

91

2353

2570

2792

3042

3294

3523

3751

3038

365

42

7

2419

2652

2877

3119

3352

3555

3751

2887

245

11

2448

2647

2851

3079

3310

3520

3728

3005

322

GA = Gestational Age (weeks), N = Number of babies in each group, C3 – C97 = Predicted Smoothed percentiles from GAMLSS model Mean & SD = Arithmetic Mean and standard deviation in each group.

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-13-38/MediaObjects/12884_2012_Article_685_Fig1_HTML.jpg
Figure 1

First born male babies smoothed centiles graph for Weight (gms) by gestational age (weeks).

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-13-38/MediaObjects/12884_2012_Article_685_Fig2_HTML.jpg
Figure 2

First born female babies smoothed centiles graph for weight (gms) by gestational age (weeks).

Table 2 shows the 3rd, 10th, 25th, 50th, 75th, 90th, 97th percentile, mean and SD birth weights for male and female babies of later born babies. Figures 3 and 4 shows the smoothed percentile curves for male and female babies of later born babies separately.
Table 2

Smoothed percentiles for birth weight (grams) of later born male and female babies

Later born male babies smoothed percentiles

Later born female babies smoothed percentiles

GA

N

C3

C10

C25

C50

C75

C90

C97

Mean

SD

N

C3

C10

C25

C50

C75

C90

C97

Mean

SD

24

11

73

199

392

645

916

1172

1440

663

102

5

55

176

383

679

1018

1353

1714

792

133

25

9

99

259

491

785

1096

1388

1692

1099

645

8

75

226

464

787

1149

1503

1882

1136

833

26

14

131

328

594

920

1260

1578

1907

914

282

9

104

287

552

895

1270

1634

2021

893

490

27

16

175

409

704

1053

1412

1745

2090

1408

909

13

147

365

652

1007

1388

1753

2142

1217

836

28

22

237

508

824

1185

1552

1892

2242

1071

316

18

211

463

766

1126

1506

1869

2253

1208

500

29

14

327

629

958

1324

1691

2031

2381

1141

416

24

302

582

894

1256

1633

1991

2369

1195

480

30

45

450

775

1107

1470

1834

2168

2513

1595

721

41

421

720

1038

1399

1772

2126

2498

1470

695

31

39

610

943

1271

1626

1980

2306

2642

1567

387

51

567

878

1197

1557

1926

2275

2642

1479

451

32

60

795

1127

1449

1795

2140

2457

2785

1820

496

51

735

1052

1373

1731

2099

2444

2807

1621

469

33

97

997

1324

1639

1978

2316

2627

2947

1928

461

60

924

1244

1564

1920

2284

2626

2984

1956

578

34

104

1214

1536

1844

2176

2507

2812

3126

2137

556

116

1140

1455

1769

2117

2472

2804

3152

2116

570

35

187

1453

1765

2064

2386

2707

3002

3307

2314

479

160

1380

1684

1984

2316

2653

2968

3298

2307

519

36

323

1712

2008

2293

2600

2906

3188

3479

2552

506

232

1637

1922

2202

2511

2824

3117

3422

2468

506

37

745

1977

2253

2518

2805

3090

3353

3625

2808

453

599

1891

2154

2412

2696

2983

3251

3530

2692

433

38

1518

2210

2466

2713

2979

3244

3489

3741

2983

395

1399

2111

2355

2595

2857

3122

3369

3626

2867

396

39

2145

2369

2615

2852

3108

3363

3598

3841

3119

382

2164

2273

2505

2732

2980

3231

3465

3707

2981

377

40

2140

2445

2692

2930

3187

3443

3680

3924

3187

390

2160

2369

2595

2816

3058

3302

3529

3765

3078

366

41

475

2471

2725

2969

3233

3496

3739

3990

3205

421

507

2409

2632

2850

3090

3331

3555

3788

3040

370

42

53

2480

2742

2994

3266

3537

3788

4046

3080

391

57

2429

2651

2869

3106

3346

3569

3801

3019

404

GA = Gestational Age (weeks), N = Number of babies in each group, C3 – C97 = Predicted Smoothed percentiles from GAMLSS model Mean & SD = Arithmetic Mean and standard deviation in each group.

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-13-38/MediaObjects/12884_2012_Article_685_Fig3_HTML.jpg
Figure 3

Later born male babies smoothed centiles graph for weight (gms) by gestational age (weeks).

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-13-38/MediaObjects/12884_2012_Article_685_Fig4_HTML.jpg
Figure 4

Later born female babies smoothed centiles graph for weight (gms) by gestational age (weeks).

Table 3 presents the agreement between the model (adjusted) and actual data (unadjusted) percentiles for first born and later born male and female babies.
Table 3

Agreement between unadjusted and adjusted centiles

  

Adjusted centiles

    
  

< 10th

10 – 90th

> 90th

Weighted kappa

P value

First born male babies: (N = 1217)

      

Unadjusted centiles

< 10th

105

8

0

  

10 – 90th

20

943

18

0.883

< 0.001

> 90th

0

6

117

  

Later born male babies: (N = 8739)

      

Unadjusted centiles

< 10th

810

43

0

  

10 – 90th

102

6791

101

0.917

< 0.001

> 90th

0

25

867

  

First born female babies: (N = 1121)

      

Unadjusted centiles

< 10th

98

8

0

  

10 – 90th

12

874

16

0.897

< 0.001

> 90th

0

6

107

  

Later born female babies: (N = 8353)

      

Unadjusted centiles

< 10th

787

10

0

  

10 – 90th

89

6551

65

0.934

< 0.001

 

> 90th

0

37

814

  

Agreement of centiles for male & female babies among the first born babies

When considering first born male babies, there is significant agreement between adjusted and unadjusted percentile classification (k=.883, p<.001). However, 7% of the male babies in <10th percentile category was misclassified as 10–90th percentile category. Nearly 2% of the babies from 10–90th percentile category was misclassified as <10th and >90th percentile. Nearly 5% of the babies >90th percentile category was misclassified as 10–90th percentile. When considering female babies, there is a significant substantial agreement between the adjusted and unadjusted classifications (k=.897, p<.001). The misclassifications to the next category percentiles rates were the same as in the male babies.

Agreement of centiles for male & female babies within the later born group

There is a significant agreement between adjusted and unadjusted percentile classification (k=.917, p<.01) in later born male babies. However, 5% of the male babies in < 10th percentile category was misclassified as 10–90th percentile category and 1.5% from 10–90th percentile category was misclassified as <10th and >90th percentile category. 2.8% of the >90th percentile category was misclassified as 10–90th percentile category. Similarly, among the later born female babies, there is a significant agreement between adjusted and unadjusted percentile classification (k=.934, p<.01). The misclassifications to the next category percentiles rates were nearly the same as among the male babies.

The mean (sd) of birthweights of the year 1996 was 2846 g (562) as compared to year 2010 (15 years later) which was 2907 (571), there was only a difference of 61 grams in the mean birthweights over one and half decades.

Discussion

This is the biggest study ever from India, dealing with nearly 20,000 deliveries, including normal mothers with no antenatal risk factors from the same hospital covering 15 years. In addition to this, a major advantage is the reliance on early ultrasound-based estimates of gestational ages and appropriate statistical modelling using Box-Cox t distribution to get over the skewness of birth weight distribution and cubic spline smoothing. Therefore, we have established standards for first born and later born mothers for male and female babies separately. Some of the earlier standards have not done these adjustments [1, 11] and some of the standards are very old [12, 13]. The absence of downturn trend in the curves in the post term period is similar to curves reported [14] and are consistent with evidence based on early ultrasound-based gestational ages [15, 16].

This study also compared the unadjusted centiles to adjusted and smoothed centiles. With male babies of first born, 7% of the adjusted <10th percentile was misclassified as 10–90th percentile and nearly each 2% of 10–90th category was misclassified as <10th or >90th percentile category. A similar trend was obtained in female babies of first born and male and female babies of later born babies. Though there is very good agreement in general the highest misclassification rate was nearly 7.5%. Therefore the use of unadjusted percentiles may lead to unnecessary intervention and anxiety for the parents of babies whose weight fall in the range of lower and upper centiles according to adjusted centiles.

The limitation of the study is that the observations are cross-sectional. That is, birth weights of different babies were observed at different gestational ages at delivery. Ideally this has to be longitudinal in nature, that is, the same number of pregnancies and their birth weights have to be observed. Anthropometric measurements during gestation are feasible only using ultra-sound. However, the ultra-sound measurements have not been proved to be valid and reliable [17, 18]. Temporal trends toward increasing maternal weight, weight gain during pregnancy due to various socio economic changes that have been taking place in the country in the last one and half decade needs to be studied.

With economic reforms, one expects improvement in birthweights. The mean (sd) of birthweights of the year 1996 was 2846 g (562) as compared to year 2010 (15 years later) which was 2907 (571), there was a difference of only 61 grams in the mean birthweights over one and half decades.

Conclusion

New standards for birth weights of Indian newborns, both term and preterm have been established. In addition, birth weight standards based on the sex of the baby and maternal parity have also been brought out. These new standards will allow interventions to be based on Indian standards.

Consent

Written informed consent was obtained from the patient for publication of this report.

Publishing datasets

The dataset will not be available for publishing online since further research has been planned.

Declarations

Acknowledgements

I acknowledge Dr. Denny Fleming’s, (Clinical Pharmacologist Christian Medical College Vellore) help in correcting the English of main manuscript.

Authors’ Affiliations

(1)
Department of Biostatistics, Christian Medical College and Hospital
(2)
Department of Obstetrics and Gynaecology, Christian Medical College and Hospital

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  19. Pre-publication history

    1. The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2393/13/38/prepub

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