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Birth weight centiles by gestational age for twins born in south India

  • Prasanna Premkumar1Email author,
  • Belavendra Antonisamy1,
  • Jiji Mathews2,
  • Santhosh Benjamin2,
  • Annie Regi2,
  • Ruby Jose2,
  • Anil Kuruvilla3 and
  • Mathews Mathai4
BMC Pregnancy and ChildbirthBMC series – open, inclusive and trusted201616:64

https://doi.org/10.1186/s12884-016-0850-y

Received: 19 May 2014

Accepted: 16 March 2016

Published: 24 March 2016

Abstract

Background

Birth weight centile curves are commonly used as a screening tool and to assess the position of a newborn on a given reference distribution. Birth weight of twins are known to be less than those of comparable singletons and twin-specific birth weight centile curves are recommended for use. In this study, we aim to construct gestational age specific birth weight centile curves for twins born in south India.

Methods

The study was conducted at the Christian Medical College, Vellore, south India. The birth records of all consecutive pregnancies resulting in twin births between 1991 and 2005 were reviewed. Only live twin births between 24 and 42 weeks of gestation were included. Birth weight centiles for gestational age were obtained using the methodology of generalized additive models for location, scale and shape (GAMLSS). Centiles curves were obtained separately for monochorionic and dichorionic twins.

Results

Of 1530 twin pregnancies delivered during the study period (1991–2005), 1304 were included in the analysis. The median gestational age at birth was 36 weeks (1st quartile 34, 3rd quartile 38 weeks). Smoothed percentile curves for birth weight by gestational age increased progressively till 38 weeks and levels off thereafter. Compared with dichorionic twins, monochorionic twins had lower birth weight for gestational age from after 27 weeks.

Conclusions

We provide centile values of birth weight at 24 to 42 completed weeks of gestation for twins born in south India. These charts could be used both in routine clinical assessments and epidemiological studies.

Keywords

Twins Birth weight Gestational age Centiles India GAMLSS

Background

Rate of twin births is rising due to the increased use of assisted reproductive technologies in the recent years [1]. Birth weight of twins are considerably lower than singletons and associated with higher risk for adverse perinatal and infant outcomes [2, 3]. Birth weight centiles by gestational age is often used as a health indicator and to understand the natural extent of variation in birth weight. There have been many studies of twin birth weight centiles worldwide [47], but are of limited use in developing settings like India owing to the use of non-representative populations. In India, information on twins is quite limited, and most previous studies on centile curves focused only on singleton births [810]. Currently, the clinical practice is that centiles curves for singleton births are used as an estimate to evaluate twin births. However, recent studies suggest use of singleton centile curves on twins is not appropriate as twins experience different growth trajectories than singletons [11]. Moreover, several studies have recommended the development of twin specific centile curves to evaluate twin births [1215].

Twin births are further complicated by placental chorionicity. Monochorionic twins present a two to three times higher risk for adverse outcomes than dichorionic twins [16], with birth weights of monochorionic twins lower than those of dichorionic twins over the gestational ages. Further, monochorionic placentation increases the risk of serious pregnancy complications (such as twin-to-twin transfusion syndrome), congenital anomalies, growth restriction, and perinatal death [17].

Past studies have indicated that placental chorionicity should be taken into consideration in assessment of twin births [18]. The lack of centile curves specific to twins could be a limiting factor in understanding the distribution of birth weight and further evaluation of twin births. Therefore, we carried out the present study to construct gestational age specific birth weight centile curves for twins born in South India, stratified by placental chorionicity (monochorionic and dichorionic placentation).

Methods

Setting and population

This study was based on labour room records and medical records maintained by the Department of Obstetrics and Gynaecology at the Christian Medical College, Vellore. This hospital serves as a maternity centre with almost 9000 deliveries annually (during the study period, 1991–2005). It provides obstetric care to local population of Vellore city and for surrounding towns and villages and also acts as a tertiary hospital. Besides women from Vellore district, women from neighbouring districts in Tamil Nadu and adjoining states of Andhra Pradesh and Karnataka also deliver in the institution. Most pregnancies are registered early during the first or second trimesters and followed up thereafter for antenatal care.

Ethical approval of the study protocol was obtained from the Institutional Review Board (IRB: 2000-no.4481) of the Christian Medical College, Vellore. However, because of the retrospective nature of the study and data were extracted from medical records/labour room registers with no individual identifications, and hence individual informed consent was not obtained.

Study sample

The study sample included all twin pregnancies delivered at the centre between January 1, 1991 and December 31, 2005. Pregnancies in which at least one child died, or missing information on study variables were excluded from further analysis.

Study variables

Birth weight was measured immediately after birth on a Braun weighing scale to the nearest 50 g. Gestational age was determined as the number of completed weeks of gestation from the last menstrual period (LMP) to the date of birth. This was best estimated using combinations of the last menstrual period (LMP), early clinical examination and early ultrasound scans. If there was a difference between gestational age estimated from LMP and ultrasound, the ultrasound estimate was used. Placental chorionicity was diagnosed by ultrasound and confirmed by gross examination of placenta after the birth.

Statistical analysis

From an initial exploratory analysis, we found that the distribution of birth weight at extreme gestational ages was non-normal and the general pattern of relationship between birth weight and gestational age was not linear. Given these violations in the usual assumptions of regression analyses, we chose the generalized additive model for location scale and shape (GAMLSS) approach [19]. This approach is highly flexible as it relaxes the traditional distributional assumptions about normality to include even highly skewed and kurtotic distributions. It extends not only to model mean but all other parameters (standard deviation, skewness and kurtosis) of the distribution as linear, non linear or smoothing functions of explanatory variables (gestational age). In our analyses, we have used Box-Cox t (BCT) distribution for modeling birth weight as non-parametric cubic spline functions of gestational age. Model selection was based on generalized Akaike Information Criterion (GAIC) and the model with smallest value of the GAIC is selected. Worm plots were used for visual inspection of the fit of the smoothed curves and were further confirmed by superimposing the smoothed centiles on observed empirical centiles. Centile curves were obtained for the entire sample and were also constructed according to placental chorionicity. The GAMLSS package for R statistical software (version 2.13.1) was used for the analysis [20].

Results

During the study period, a total of 1673 multiple pregnancies were delivered. Of which, the following were removed sequentially from further analysis (triplets = 39; fetal deaths of one or more foetuses = 141; missing data on chorionicity = 170; and missing data on birth weight and/or gestational age = 19). Thus complete data were available from 1304 twin gestations for analyses. The mean maternal age of mothers included in the sample was 25.2 years (SD = 4.3) and 46 % of mothers were primigravid. A total of 88 (6.7 %) mothers were conceived using some form of assisted reproductive technologies, while approximately (64) 5 % of mothers experienced gestational diabetes and (224) 17 % had preeclampsia. Eighty two percent of the women in our sample were Hindus, 11 % were Muslims and 7 % were Christians. About 8 % (106) of mothers were illiterates. There were 457 (35 %) monochorionic pregnancies.

The median gestational age at birth was 36 weeks (IQR 34–38 weeks). There was a 4.1 % increase in adjusted (for gestational age) mean birth weight from 2050 g in 1991 to 2135 in 2005. Dichorionic twins were heavier than monochorionic twins with an adjusted (for gestational age) mean of 2138 g compared with 2, 054 g respectively. The mean birth weight discordance was 13.1 % (SD = 10.3 %, median = 11.1 %). Considering a threshold of 18 % [18], birth weight discordance was identified in 360 out of 1304 pregnancies (27.6 %).

Examining the 50th centile, the weekly increase in birth weight flattens by 38 weeks of gestation and thereafter gain in the median birth weight was negligible (Fig. 1). To assess the validity of the fitted model, the expected percentage of observed birth weights below each centile was compared with observed percentage across gestational ages. About, 9 % fell below the 10th centile, 80.4 % between 10th and 90th centile and 9.9 % above the 90th centile. Further, the fit of the curves estimated from the statistical models were confirmed by overlaying the empirical centiles on top of the smoothed centiles (Fig. 1).
Fig. 1

Smoothed centiles of birth weight for gestational age for the entire sample of twins (solid lines) and raw centiles (dotted lines). Points are jittered to improve readability

Figure 2 present birth weight centile curves for twins according to placental chorionicity. Monochorionic twins were consistently smaller than dichorionic twins after 27 weeks of gestation, with a fall-off across centiles of birthweight (Tables 1 and 2).
Fig. 2

Smoothed centiles of birth weight for gestational age according to chorionic presentations

Table 1

Distribution of birthweight by gestational age (weeks) for twins according to chorionic placentation

 

Monochorionic

Dichorionic

GA

N

P10

P50

P90

Mean

SD

N

P10

P50

P90

Mean

SD

24

4

600

640

708

650

60.00

2

555

575

595

575

35.36

25

8

657

725

815

733.75

71.30

2

681

725

769

725

77.78

26

8

709

840

1018

860

147.45

4

698

740

775

737.5

45.00

27

2

756

780

804

780

42.43

12

660

840

1091

889.17

215.43

28

6

830

1120

1375

1108.33

264.08

12

610

1015

1205

987.5

230.26

29

26

785

1125

1300

1076.92

195.44

16

1060

1265

1425

1243.75

176.74

30

18

1052

1270

1503

1263.89

182.92

26

1130

1285

1550

1304.23

220.53

31

18

1075

1400

1645

1387.22

275.66

50

1189

1500

1823

1504.2

275.19

32

54

1212

1550

1947

1578.89

306.84

80

1104

1580

1873

1531.62

322.79

33

54

1258

1680

1887

1633.7

303.12

84

1383

1750

2278

1820.82

380.85

34

70

1489

1922.5

2302

1944.5

332.03

136

1520

1885

2255

1891.92

306.12

35

90

1550

1950

2301

1933.89

301.72

178

1667

2025

2560

2060.86

344.94

36

134

1550

2250

2585

2143.02

416.99

236

1645

2200

2570

2164.37

391.02

37

136

1775

2240

2790

2248.53

430.99

322

1840

2350

2800

2340.81

384.96

38

146

1895

2400

2895

2419.79

397.78

276

1985

2450

3040

2469.31

409.96

39

82

1830

2350

3000

2349.88

419.67

192

1825

2500

3094

2485.16

470.86

40

42

1924

2300

2886

2364.52

423.32

44

2109

2565

3024

2576.82

407.95

41

16

1395

1960

2650

2013.75

553.98

16

1925

2470

2895

2410

429.33

       

6

1960

2850

3030

2613.33

524.96

P10 10th centile, P50 50th centile, P90 90th centile

Table 2

Smoothed birthweight (g) centiles by gestational age (weeks) for twins according to chorionic placentation

  

Monochorionic

 

Dichorionic

GA

N

P5

P10

P25

P50

P75

P90

P95

N

P5

P10

P25

P50

P75

P90

P95

24

4

435

478

550

627

702

768

807

2

337

391

461

524

587

657

713

25

8

503

553

636

725

812

889

934

2

426

490

574

652

729

813

877

26

8

570

628

721

823

921

1008

1059

4

516

589

687

779

870

967

1040

27

2

637

701

806

919

1029

1126

1183

12

610

692

804

910

1015

1125

1205

28

6

707

778

894

1019

1141

1249

1312

12

709

799

924

1045

1165

1288

1376

29

26

782

861

990

1129

1264

1383

1453

16

810

908

1047

1184

1319

1455

1551

30

18

873

961

1104

1259

1409

1542

1621

26

914

1020

1173

1324

1475

1624

1728

31

18

974

1073

1233

1405

1574

1722

1810

50

1020

1134

1301

1468

1633

1796

1907

32

54

1081

1190

1367

1559

1746

1911

2008

80

1128

1250

1431

1613

1794

1971

2089

33

54

1187

1307

1502

1712

1917

2098

2205

84

1238

1368

1563

1761

1957

2148

2274

34

70

1293

1424

1636

1865

2089

2286

2402

136

1347

1485

1693

1907

2119

2323

2457

35

90

1393

1533

1762

2009

2250

2462

2587

178

1454

1600

1821

2050

2277

2494

2635

36

134

1489

1639

1883

2147

2404

2631

2765

236

1557

1709

1943

2187

2428

2657

2805

37

136

1569

1727

1985

2263

2535

2774

2915

322

1653

1812

2057

2314

2569

2809

2963

38

146

1624

1788

2055

2343

2624

2872

3018

276

1739

1904

2160

2428

2695

2946

3105

39

82

1647

1815

2087

2381

2666

2919

3069

192

1817

1987

2251

2531

2808

3068

3232

40

42

1646

1819

2096

2391

2680

2937

3090

44

1890

2064

2337

2626

2913

3181

3349

41

16

1613

1801

2091

2391

2684

2952

3119

16

1960

2138

2419

2718

3014

3290

3462

42

0

       

6

2028

2210

2499

2807

3113

3396

3573

P5 5th centile, P10 10th centile, P25 25th centile, P50 50th centile, P75 75th centile, P90 90th centile, nd P95 95th centile

Discussion

In this study, we constructed new birth weight centile curves for twins born in South India. We have presented centile curves by chorionic placentation to facilitate consideration of chorionicity in the assessment of twin births.

The overall pattern of change in birthweight over gestational age was characterized by a rapid change in weight till 38 weeks and reduction in change then onwards. Given that it is increasingly possible to determine chorionicity prenatally, it is important to consider placental chorionicity in the assessment of growth in twins. Our comparison of centile curves by chorionicity showed that birth weights of monochorionic twins were lower than dichorionic twins in gestational ages between 28 and 42 weeks. This could be explained by the increased demands with advancing gestational age in monochorionic twins which share a common placenta and this heightened demand may not be met as adequately as in dichorionic twins- leading to the difference between two groups.

Previous studies on distribution of birth weights in India have mainly been based on singleton births. Birth weights from our study were consistently lower than those of singletons [8]; the differences were approximately 500 g between gestational ages 32 and 42 weeks. This difference was similar to that seen in other published studies on the birth weight centile curves for twins [4, 6].

The data presented here is based on the largest sample size reported till date from India. However, in developing settings like India, it is considerably difficult to obtain precise obstetrical records on measurements at birth for a large number of twins, as there are not many population based twin registries. The new birth weight curves may provide useful evidence for better understanding the birth weight of twins born in South India. For instance, it could serve as a useful tool for clinicians to evaluate and assess the birth weight of newborn twins. Additionally, this new centile curves should be a useful for epidemiologic research on twins related to determination of geographic differences, temporal trends and etiologic determinants of distribution of birth weight.

One of some limitations of this study is that the data were drawn from a tertiary care hospital, and hence it may restrict the generalizability of our results. However, given that twin pregnancies are considered as high-risk and often referred to tertiary care hospitals, the problems related to generalizability might be less likely. Another limitation is the measurement of gestational age using dates of last menstrual period, which suffers from recall bias. We believe our estimates are likely to be improved with the use of early ultrasound to correct estimates of gestational age. Also, data on birthweight and placental chorionicity captured during the course of routine clinical care may not be as precise as measurements under more controlled research settings. Thus, for example, we were not able to ascertain the extent of intra or inter observer variability. Additionally, the number of infants in extreme gestational ages was not sufficiently larger to enable accurate estimation of centiles. Further, in our study, the inclusion criteria resulted in a more general reference for birth weight, describing the variation in birth weight within a reference population and did not delineate variation that can be considered ‘ideal’ or of ‘desirable targets’. Despite these limitations, our study will add to the existing scanty literature on birth weight distributions for twin births and will provide basis for future epidemiological studies on twins from this region.

Conclusion

The use of population specific birth weight centile curves will better aid both the clinician and researcher in the assessment of the birth weight of twins. Further, we recommend that assessment in twins consider placental chorionicity. The charts will provide a benchmark to examine the birth weight of twins in relation to other twins born of same gestational age, and would serve as a baseline for future epidemiological research studies. Future work will be to assess whether the infants identified in this way are those with high risk for poor perinatal outcomes, such as stillbirth and neonatal death.

Abbreviation

GAMLSS: 

generalized additive models for location scale and shape

Declarations

Acknowledgements

We acknowledge Nithya Jeyaseeli and Regina Varghese for their assistance with the data collection.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Departments of Biostatistics, Christian Medical College
(2)
Obstetrics and Gynaecology, Christian Medical College
(3)
Neonatology, Christian Medical College
(4)
Making Pregnancy Safer Department, World Health Organization

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Copyright

© Premkumar et al. 2016