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Centile charts for birthweight for gestational age for Scottish singleton births

  • Sandra Bonellie1Email author,
  • James Chalmers2,
  • Ron Gray3,
  • Ian Greer4,
  • Stephen Jarvis5 and
  • Claire Williams1
BMC Pregnancy and Childbirth20088:5

DOI: 10.1186/1471-2393-8-5

Received: 07 September 2007

Accepted: 25 February 2008

Published: 25 February 2008

Abstract

Background

Centile charts of birthweight for gestational age are used to identify low birthweight babies. The charts currently used in Scotland are based on data from the 1970s and require updating given changes in birthweight and in the measurement of gestational age since then.

Methods

Routinely collected data of 100,133 singleton births occurring in Scotland from 1998–2003 were used to construct new centile charts using the LMS method.

Results

Centile charts for birthweight for sex and parity groupings were constructed for singleton birth and compared to existing charts used in Scottish hospitals.

Conclusion

Mean birthweight has been shown to have increased over recent decades. The differences shown between the new and currently used centiles confirm the need for more up-to-date centiles for birthweight for gestational age.

Background

Birthweight is one of the important indicators used to assess the health of an infant at birth. Low birthweight has often been defined as weights less than 2500 grams with birthweights less than 1500 grams classed as very low birthweight. These definitions however do not take into account gestational age. It is important to be able to differentiate between babies who are light because they are premature and those who are small-for-gestational age since the latter may have different health problems to the former. They may be growth restricted and have an increased risk of other complications such as perinatal asphyxia, symptomatic hypoglycaemia, congenital malformations, chronic intra-uterine infection and pulmonary haemorrhage [1]. Large-for-gestational age babies also have related health problems. Identification of small or large for gestational age babies is important for the management of the individual pregnancy and neonate. It is also a valuable aid to epidemiological studies where the aim is to identify risk factors or to assess the management of pregnancies [2].

Small- or large-for-gestational age babies may be identified using centile charts of birthweight by gestational age. Centile reference charts are used to monitor clinical measurements on individuals in the context of population values. Raw centiles can be calculated from appropriate data but the perturbations in these curves are unlikely to reflect the pattern of underlying growth at the population level. It is therefore reasonable to use statistical methods to derive a series of smoothed curves showing how the centiles of a measurement, in this case birthweight, change when plotted against time, in this case gestational age.

In Scotland there have been three sizeable studies resulting in the production of centile charts each based on data collected in Aberdeen [35]. The charts from the most recent of these studies, using data from 17,927 singleton births occurring between 1975–1980, were extensively used as a standard throughout Scotland until relatively recently.

The Information Services Division (ISD) of the Scottish Health Service use and publish birthweight centile charts[6]. ISD collects data on all maternity patients admitted to Scottish hospitals on an SMR02 form. The charts are based on 894,066 live births occurring between 1975 and 1989 and are the most recent published in Scotland.

Increases in birthweight since the formation of these standards have been observed for Scotland [7] England and Wales [8], the United States [9] and Canada [10] There have also been changes in the methods used to calculate gestational age [11]. These changes suggest that the centile charts in present use may now be inaccurate. Therefore, we aimed to produce updated charts using more recent data from 1998–2003.

Methods

Data on singleton births occurring between 1980 and 2003 were obtained from ISD's SMR02 (maternity) data collection system. This includes information on the birthweight, gestational age and sex of the infant. The parity of the mother is also recorded. Gestational age at birth was reported in completed weeks and is a clinician's estimate of gestation at birth based on an ultrasound dating scan and date of last menstrual period.

In order to adequately represent the population of all singleton births, the only exclusions made were lethal congenital anomalies and obvious outliers which included any birthweights less than 250 grams. Outliers were identified using Tukey's methodology [12]. This calculates the interquartile range and identifies as outliers any values more than twice the interquartile range below the first quartile or above the third quartile. This method assumes a symmetric distribution which is not the case for birthweights at most gestational ages. However the values of L obtained in each of the groups for each suggests only a slight degree of skewness at most gestational ages. The number of birthweights omitted as possible outliers was small and inspection of the omitted birthweights suggests that most of these could be explained by transcription errors.

The mean birthweight of all singletons born in each year between 1980–2003 was calculated. This confirmed the reported increase in birthweight over this period. This increase is marked over the period from 1980–1997 but appears to level off from 1998 onwards and therefore the most recent years for which complete data were available, namely 2002 and 2003, were used as a basis from which to construct new centile charts. For births occurring at gestations between 31 and 42 weeks a two year period gives sufficient data, however for the extremes of gestational age the data was supplemented by births from 1998 to 2001.

Centiles were calculated using the LMS method [13] which uses the Box-Cox power transformation to obtain normally distributed data within each group. This involves estimating three sets of values for each gestational age group, namely, L the power transformation used to achieve normality, M the median birthweight and S the coefficient of variation of the data. L, M and S are estimated for each gestational age and then smoothed curves are fitted using cubic splines to these to give L(t), M(t) and S(t) where t is the gestational age. The extent of the smoothing is expressed in terms of the degrees of freedom used for the fit. The 100αth centile for the appropriate sex and parity group is then given by

C100α (t) = M(t) [1+L(t)S(t)Z α ]1/L(t)

where Z α is the α % point of the normal distribution.

For a particular infant, with birthweight y, a z-score can be calculated using the formula
z = [ y M ( t ) ] L ( t ) 1 L ( t ) S ( t ) MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacPC6xNi=xI8qiVKYPFjYdHaVhbbf9v8qqaqFr0xc9vqFj0dXdbba91qpepeI8k8fiI+fsY=rqGqVepae9pg0db9vqaiVgFr0xfr=xfr=xc9adbaqaaeGacaGaaiaabeqaaeqabiWaaaGcbaGaeeOEaONaeyypa0tcfa4aaSaaaeaadaWadaqaamaalaaabaGaeeyEaKhabaGaeeyta00aaeWaaeaacqqG0baDaiaawIcacaGLPaaaaaaacaGLBbGaayzxaaWaaWbaaeqabaGaeeitaW0aaeWaaeaacqqG0baDaiaawIcacaGLPaaaaaGaeyOeI0IaeGymaedabaGaeeitaW0aaeWaaeaacqqG0baDaiaawIcacaGLPaaacqqGtbWudaqadaqaaiabbsha0bGaayjkaiaawMcaaaaaaaa@4526@

Four sets of charts were constructed defined by the sex of the baby, male or female, and the parity of the mother, nulliparous or multiparous. Centiles were calculated using the software LMS ChartMaker. Other analysis was carried out using SAS, version 9.1

Results

The mean birthweight for each of the years from 1980 to 2003 is shown in Figure 1 and confirms the previously reported increase in birthweight.
https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig1_HTML.jpg
Figure 1

Mean birthweight by year.

There were 98,904 records of singleton births occurring in 2002 and 2003. These were supplemented by information on 1,883 singleton births from 1998–2001 for gestational ages of 30 weeks or less or 43 weeks. Excluding lethal congenital anomalies and omitting outliers gave a total of 100,133 records. Applying Tukey's method resulted in 0.4% of the observations being omitted as outliers. Figures 2a and 2b show plots of birthweight against gestational age with and without the outliers for the subgroup girls, parity 1 or more. Table 1 gives the numbers of births used in constructing the centiles, and the percentage of outliers omitted together with the overall mean birthweight and standard deviation based on the data for 2002–2003 only.
https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig2_HTML.jpg
Figure 2

a: Birthweight by gestational age for girls, parity 1 or more. b: Birthweight by gestational age for girls, parity 1 or more with outliers removed.

Table 1

Summary of Data by Sex and Parity Groupings

Group

Total Numbers

2002–2003 Data

Sex of infant

Parity

Number of births used

Percentage of outliers

Mean(St.Dev) Birthweight (with outliers omitted)

Male

0

23419

0.37

3376 (603.33)

 

1 or more

27924

0.43

3494 (603.03)

Female

0

21948

0.39

3266 (570.94)

 

1 or more

26842

0.37

3369 (570.79)

Tables 2, 3, 4, 5 give the centiles for the groups: boys parity 0, boys parity 1 or more, girls parity 0 and girls parity 1 or more respectively. The tables also give the number of births used and the fitted values of L. M and S for each gestational age for each group, as well as the degrees of freedom used in fitting the cubic splines.
Table 2

Centiles for Boys, Nulliparous

Gestational Age

No.

L d.f. = 5

M d.f. = 12

S d.f. = 6

3rd

5th

10th

25th

50th

75th

90th

95th

97th

24

65

1.30

658

0.245

326

372

440

546

658

764

856

910

944

25

59

1.34

759

0.240

379

432

510

632

759

879

982

1042

1080

26

101

1.38

851

0.235

430

490

577

712

851

982

1095

1160

1202

27

105

1.41

958

0.229

494

561

656

805

958

1101

1224

1295

1341

28

128

1.43

1103

0.222

585

659

766

932

1103

1263

1400

1479

1530

29

143

1.44

1271

0.214

696

778

896

1081

1271

1449

1601

1689

1745

30

160

1.43

1446

0.205

823

911

1039

1239

1446

1640

1807

1904

1966

31

86

1.41

1643

0.196

976

1068

1204

1419

1643

1855

2037

2143

2211

32

105

1.38

1848

0.187

1142

1239

1382

1609

1848

2075

2272

2387

2460

33

121

1.34

2065

0.178

1326

1425

1574

1812

2065

2308

2519

2643

2722

34

213

1.27

2286

0.169

1521

1622

1774

2021

2286

2543

2768

2901

2986

35

341

1.17

2510

0.161

1729

1830

1983

2235

2510

2779

3018

3159

3250

36

586

1.05

2744

0.153

1950

2050

2204

2461

2744

3026

3278

3428

3526

37

1051

0.93

2959

0.145

2159

2259

2412

2671

2959

3250

3513

3671

3774

38

2447

0.84

3162

0.137

2363

2461

2613

2871

3162

3457

3726

3889

3996

39

4459

0.77

3341

0.130

2546

2643

2794

3050

3341

3638

3910

4075

4182

40

6421

0.74

3510

0.125

2711

2809

2960

3217

3510

3809

4083

4250

4359

41

5906

0.76

3664

0.120

2859

2957

3110

3369

3664

3964

4238

4405

4514

42

867

0.83

3736

0.116

2935

3034

3187

3445

3736

4031

4299

4461

4567

43

55

0.92

3764

0.112

2976

3074

3225

3479

3764

4050

4309

4465

4566

Table 3

Centiles for Boys, Multiparous

Gestational Age

No.

L d.f. = 4

M d.f. = 15

S d.f. = 8

3rd

5th

10th

25th

50th

75th

90th

95th

97th

24

61

1.24

628

0.229

339

378

436

529

628

723

806

855

886

25

59

1.23

756

0.224

418

463

531

640

756

868

966

1024

1061

26

71

1.22

866

0.218

490

540

615

736

866

991

1101

1166

1207

27

72

1.21

996

0.212

578

633

716

851

996

1137

1260

1333

1380

28

111

1.20

1147

0.207

679

740

833

984

1147

1305

1444

1526

1579

29

122

1.18

1308

0.203

788

856

959

1126

1308

1484

1640

1732

1791

30

153

1.15

1483

0.200

907

982

1096

1281

1483

1681

1856

1959

2026

31

62

1.10

1676

0.196

1044

1125

1249

1453

1676

1897

2093

2209

2284

32

100

1.05

1859

0.192

1183

1269

1400

1618

1859

2099

2314

2442

2525

33

135

0.98

2065

0.187

1344

1434

1573

1806

2065

2325

2560

2701

2792

34

209

0.90

2284

0.181

1520

1614

1760

2007

2284

2565

2821

2975

3076

35

312

0.82

2523

0.178

1708

1807

1961

2224

2523

2828

3108

3278

3389

36

679

0.75

2792

0.172

1927

2031

2194

2473

2792

3121

3425

3610

3731

37

1448

0.70

3063

0.160

2181

2287

2452

2737

3063

3400

3711

3902

4027

38

3940

0.69

3313

0.144

2457

2560

2721

2997

3313

3639

3940

4124

4245

39

6247

0.70

3480

0.130

2663

2762

2916

3179

3480

3788

4072

4245

4358

40

7809

0.73

3649

0.123

2831

2931

3086

3349

3649

3955

4236

4407

4519

41

5665

0.76

3793

0.120

2962

3063

3221

3489

3793

4102

4386

4557

4670

42

629

0.79

3856

0.120

3005

3110

3272

3546

3856

4172

4460

4634

4748

43

40

0.83

3866

0.123

2987

3095

3263

3547

3866

4190

4486

4665

4781

Table 4

Centiles for Girls, Nulliparous

Gestational Age

No.

L d.f. = 4

M d.f. = 12

S d.f. = 6

3rd

5th

10th

25th

50th

75th

90th

95th

97th

24

55

1.43

604

0.254

270

319

389

496

604

704

789

838

869

25

56

1.35

682

0.252

320

372

446

562

682

794

891

947

983

26

78

1.27

779

0.249

382

437

517

645

779

907

1018

1084

1125

27

69

1.19

888

0.246

453

511

598

738

888

1033

1160

1235

1283

28

118

1.12

1018

0.241

540

602

696

850

1017

1181

1327

1413

1468

29

102

1.06

1173

0.234

648

715

818

987

1173

1357

1522

1620

1684

30

157

1.01

1339

0.226

770

842

952

1136

1339

1543

1726

1836

1907

31

50

0.97

1515

0.216

904

980

1097

1294

1514

1735

1935

2055

2133

32

88

0.94

1709

0.206

1057

1138

1263

1473

1709

1947

2163

2293

2377

33

118

0.92

1927

0.195

1233

1319

1451

1675

1927

2181

2412

2551

2642

34

161

0.91

2160

0.183

1429

1519

1659

1894

2159

2428

2672

2819

2914

35

303

0.89

2399

0.172

1640

1734

1879

2123

2399

2678

2932

3086

3186

36

462

0.87

2633

0.161

1855

1950

2099

2350

2633

2920

3182

3340

3443

37

955

0.85

2851

0.150

2066

2162

2312

2565

2851

3142

3407

3567

3672

38

2183

0.80

3062

0.140

2280

2376

2524

2776

3062

3353

3619

3780

3886

39

4240

0.73

3230

0.131

2464

2557

2702

2949

3230

3518

3783

3944

4049

40

6223

0.65

3371

0.125

2610

2702

2845

3091

3371

3661

3928

4091

4198

41

5718

0.57

3514

0.121

2754

2845

2987

3232

3514

3806

4078

4244

4354

42

760

0.49

3590

0.117

2845

2934

3073

3313

3590

3879

4148

4314

4424

43

52

0.41

3630

0.113

2909

2994

3128

3360

3630

3912

4176

4339

4447

Table 5

Centiles for Girls, Multiparous

Gestational Age

No.

L d.f. = 4

M d.f. = 14

S d.f. = 8

3rd

5th

10th

25th

50th

75th

90th

95th

97th

24

65

1.14

630

0.247

326

366

426

524

630

734

826

881

916

25

59

1.11

720

0.245

377

422

489

599

719

837

942

1004

1044

26

101

1.09

832

0.244

442

492

568

694

832

968

1089

1161

1208

27

105

1.06

958

0.241

517

573

659

802

958

1114

1253

1335

1389

28

128

1.04

1073

0.235

595

656

748

902

1072

1242

1394

1484

1543

29

143

1.01

1222

0.225

706

770

870

1037

1222

1407

1574

1673

1738

30

160

0.96

1406

0.214

846

916

1023

1204

1406

1609

1793

1903

1975

31

86

0.92

1580

0.203

989

1061

1174

1365

1580

1798

1996

2115

2193

32

105

0.87

1759

0.193

1137

1213

1331

1531

1759

1990

2201

2329

2412

33

121

0.82

1956

0.184

1301

1381

1505

1716

1956

2202

2427

2564

2654

34

213

0.77

2188

0.177

1488

1572

1704

1930

2188

2453

2698

2847

2945

35

341

0.72

2442

0.173

1684

1775

1917

2161

2442

2732

3000

3164

3272

36

586

0.68

2687

0.169

1877

1973

2125

2386

2687

2999

3289

3466

3583

37

1051

0.65

2932

0.161

2092

2192

2349

2620

2932

3256

3558

3742

3864

38

2447

0.63

3176

0.146

2347

2446

2601

2868

3176

3495

3792

3974

4094

39

4459

0.62

3352

0.132

2556

2652

2801

3057

3352

3656

3939

4111

4225

40

6421

0.64

3498

0.126

2706

2802

2951

3206

3498

3799

4077

4247

4359

41

5906

0.69

3625

0.122

2824

2921

3072

3330

3625

3927

4205

4375

4486

42

867

0.73

3673

0.122

2855

2954

3109

3373

3673

3979

4260

4431

4543

43

55

0.78

3669

0.126

2821

2924

3085

3359

3668

3983

4271

4446

4560

The z-scores resulting from the LMS models fitted should be normally distributed within each grouping. This was verified by obtaining normal probability plots of the z-scores overall and for each gestational age. The plot for girls, parity 1 or more is given in Figure 3. Table 6 gives the observed percentage of z-scores by centiles groupings for the same group.
https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig3_HTML.jpg
Figure 3

Q-Q plot of z-scores for girls, parity 1 or more.

Table 6

Percentage of observations (observed and expected) within centile bands

Centiles

Expected Percentage

Observed Percentage

Less than 3rd

3

3.1

Between 3rd and 5th

2

1.9

Between 5th and 10th

5

4.8

Between 10th and 25th

15

14.9

Between 25th and 50th

25

25.5

Between 50th and 75th

25

25.1

Between 75th and 90th

15

14.5

Between 90th and 95th

5

5.0

Between 95th and 97th

2

2.0

Above 97th

3

3.2

In order to assess the goodness of fit of the models, the new centiles were plotted against the observed centiles for each group. Figure 4 shows this plot for the 3rd, 50th and 97th centiles for girls, parity 1 or more. Figure 5 shows the 3rd, 10th, 25th, 50th, 75th, 90th and 97th centiles superimposed on the actual birthweights for the same groups. For comparison Figure 6 shows the new centiles compared to the currently used ISD centiles again for the 3rd, 50th and 97th centiles
https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig4_HTML.jpg
Figure 4

New centiles vs observed centiles for girls, parity 1 or more. 3rd, 50th and 97th centiles. Solid line: new centiles. Dashed line: observed centiles.

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig5_HTML.jpg
Figure 5

New centiles with birthweights for girls, parity 1 or more. 3rd, 10th, 25th, 50th, 75th, 90th, 97th centiles.

https://static-content.springer.com/image/art%3A10.1186%2F1471-2393-8-5/MediaObjects/12884_2007_Article_5_Fig6_HTML.jpg
Figure 6

New centiles vs ISD centiles for girls, parity 1 or more. 3rd, 50th and 97th centiles. Solid line: new centiles. Dashed line: ISD centiles.

Discussion

Centile charts of birthweight for gestational age are a valuable tool in many epidemiological studies as well as providing important information to clinicians as to which babies may be at higher risk of neonatal or postnatal morbidity [14]. It is therefore essential that the charts used are representative of the population to which they are applied. A number of standards are available based on births occurring in various European countries; mostly using data from the 1980s and the 1990s [15].

There are clear differences between the centiles calculated here from recent data and those in current use in Scotland which are based on data from 1975–1989. For term babies the median birthweight in all sex and parity groupings is shown to be higher than it was previously. This increase in birthweight is also reflected in the other centiles. For babies born at very low gestational ages the median birthweight is now less than it was, possibly reflecting the increased survival rate in pre-term births [16]. The centiles for lower gestational ages are also much closer together than in the existing charts. One possible explanation for such a marked difference at lower gestations in particular may be poor estimation of gestational age, particularly in the 1970s, as was found in data for England and Wales analysed by Milner and Richards in 1974 [17].

In recent years a number of centiles charts [18] have been constructed using the method developed by Gardosi [19]. This method aims to give a fetal weight standard and requires only data for term births from the population of interest. Whilst it is desirable in principle to look at fetal weights the assumptions which are being made with this method cannot be substantiated with reference to our data which consists only of actual birthweights. It is therefore not possible to assess the goodness of fit of the centiles calculated in this way.

As well as modelling the median birthweight the LMS method also models the coefficient of variation S and the power L which is used to transform the birthweights to achieve normality. Within each of the sex and parity groupings it is seen that the coefficient of variation decreases with increased gestational age showing that the birthweights are more variable at lower gestational ages. This contrasts with the assumption used in Gardosi's methodology for fetal weights that the coefficient of variation is constant.

It is important in constructing charts of this type to test the adequacy of the model fitted both with reference to the raw data used to construct the charts and to the assumptions on which the model relies [20, 21].

Comparing the new centiles to the empirical centiles suggests that the LMS method is a reasonable fit to the data. It can be shown that, in general, the standard errors of empirical centiles are larger than those for the centiles calculated using the normal distribution. The latter method is therefore more efficient. This is only true if the assumption of normality is reasonable which is not the case for birthweight and therefore some transformation of the data is required. A value of 1 for L indicates no transformation required with a value less than 1 adjusting for positive skewness and a value greater than 1 for negative skewness. For each sex and parity grouping the values of L suggest that the birthweights are negatively skewed for low gestational ages and positively skewed for higher gestational ages. The values of L suggest the extent of skewness at each gestational age is not high.

Normal probability plots of the z-scores for each grouping and for each gestational age within each grouping show that the LMS method has largely succeeded in achieving normality. There is some suggestion in the plot of heavier tails however the percentages in the tails are close to what is expected.

An important question in constructing centile charts of any data is which cases to include in the calculations and which to omit. Many previous studies into centile charts have used live births only because of the difficulty of accurately assessing the gestational age of stillbirths. The argument in the past has been that a baby which is stillborn may have died some time before delivery and therefore the weight may not be a true reflection of the gestational age at which delivery occurs. This is not often the case now. Fetal death is almost always recognized very quickly, and most women prefer to be delivered as soon as possible once it is realized that this has happened. This was argued by Tin[16] looking at the problems of estimating centiles for babies born before 32 weeks gestational age, In this paper it was suggested that not all stillbirths should be excluded, arguing that by doing so centiles at gestational ages less than 28 weeks have been largely overestimated.

For babies born within ten weeks of term the difference in centiles including and excluding stillbirths are negligible because the numbers of stillbirths are relatively small. Omitting stillbirths at low gestations of 24–27 weeks gestation causes bias in the centiles possibly because very small babies at any specified gestation are much more likely to be treated as "effectively" stillborn than larger babies of the same gestation when pregnancy ends as soon as this.

Information on ethnicity is poorly recorded on the SMR02 forms therefore no attempt was made to produce separate centiles for different ethnic groups. From the 2001 census it is known that the minority ethnic population was just over 100,000 in that year which is 2% of the total population of Scotland. The percentage is similar for women of child bearing age. Ethnicity is not therefore a major consideration for the Scottish data.

Other studies have followed the convention of excluding babies with major congenital malformations [22] and this has been used in this study. However with such a large data set the exclusion has made little difference to the centiles. Other studies [1, 23] have also identified outliers at each gestational age using the criterion outlined by Tukey. From visual inspection of the charts with and without the outliers, it is clear that the points identified in this study are most likely to be due to transcription errors. The excluded points do not therefore raise any concerns about the accuracy with which gestational age is measured.

Other factors are known to have a significant effect on birthweight and a number of customised charts have been developed in recent years. It can be desirable to take into account physiological factors such as the height of the mother which contribute to the natural variation in birthweights but not potential risk factors such as whether or not the mother smokes. The distinction between the two types of factor may not always be clear cut however. For example height and weight of the mother may in part be determined by risk factors such as social deprivation or nutrition. There is therefore an important role in epidemiological studies into adverse perinatal outcomes for charts such as the ones described here which will allow both the effect of infant's size and the size of the mother to be separated.

Conclusion

The differences shown between the new centiles and the current published centiles confirm the need to have centiles appropriate for the population for which the charts are to be used. Use of inappropriate centiles may result either in small-for-dates babies not being identified or too many babies being flagged as small-for-dates. After consistent increases in mean birthweight from 1980 until the mid 1990s, mean birthweight has stabilised over recent years making the new charts appropriate for current use. It is however important that the distribution of birthweight continues to be monitored on a regular basis.

Declarations

Acknowledgements

This work was funded by the Chief Scientists Office. We thank Edmund Hey for his contribution and valuable comments. Dr. Gray is funded by a core grant from the Department of Health in England to the National Perinatal Epidemiology Unit. The views expressed in this paper are those of the authors and do not necessarily reflect the views of the Department of Health.

Authors’ Affiliations

(1)
School of Accountancy, Economics and Statistics, Napier University
(2)
ISD, Gyle Square
(3)
NPEU, University of Oxford
(4)
Hull York Medical School, University of York
(5)
Sir James Spence Institute of Child Health, Royal Victoria Infirmary

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  24. Pre-publication history

    1. The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2393/8/5/prepub

Copyright

© Bonellie et al; licensee BioMed Central Ltd. 2008

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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