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Australian national birthweight percentiles by sex and gestational age, 1998–2007

Timothy A Dobbins, Elizabeth A Sullivan, Christine L Roberts and Judy M Simpson
Med J Aust 2012; 197 (5): 291-294. || doi: 10.5694/mja11.11331
Published online: 3 September 2012

Birthweight remains one of the strongest predictors of infant mortality and morbidity,1,2 and it has been established that assessing an infant’s birthweight requires summary data that account for gestational age.3 Birthweight percentiles form a reference incorporating weight and gestational age of infants at birth and are used as an adjunct for detecting neonates at higher risk of neonatal and postneonatal morbidity and growth impairment. A birthweight small or large for gestational age, often defined as being lower than the 10th percentile or higher than the 90th percentile, provides an indication of risk of perinatal morbidity and mortality.4

Australia’s first birthweight percentiles based on national population data were published in 1999.5 The period since this publication has seen numerous changes in maternal characteristics:

  • Maternal age has increased, with the proportion of mothers aged 35 years and over increasing from 15.7% in 1998 to 22.3% in 2007.6,7

  • The proportion of women reporting smoking during pregnancy has reduced from 18.4% in 20028 to 16.6% in 2007;7 with the proportion in New South Wales reducing from 22.0% in 1994 to 13.8% in 2005.9

  • Maternal overweight and obesity has increased in Australia, as it has in other developed countries,10 from 34% in the 1998–2002 calendar years11 to 50% in 2008.12 Mothers who are overweight or obese are more likely to have an indicated preterm birth, with infants born to overweight or obese women more likely to weigh more than 4 kg.13

  • The ethnicity of mothers in Australia has changed, with 22.0% in 1998 reporting that they were not born in Australia (7.4% were Asian born),6 compared with 24.3% in 2007 (9.6% Asian born).7

Identification of babies who are small or large for gestational age is important for clinical management,14 and Australian birthweight percentiles continue to be used for both clinical and research purposes.15,16 Current national birthweight percentiles are therefore required.

Our study presents national birthweight percentiles for all male and female singleton infants born in Australia over the 10-year period between 1998 and 2007.

Methods

We obtained data on singleton live births between 1998 and 2007 from the National Perinatal Data Collection (NPDC) of the Australian Institute of Health and Welfare (AIHW) National Perinatal Statistics Unit. Information is included in the NPDC for all births in Australia of at least 400 g birthweight or at least 20 weeks’ gestation. Birthweight is recorded to the nearest 5 g, with gestational age recorded in completed weeks based on the first day of the last menstrual period or the best available clinical estimate (including early pregnancy ultrasound examination).

We excluded implausible birthweights using a method based on Tukey’s box-and-whisker plots.17 For each sex and gestational age combination, birthweights below the first quartile minus twice the interquartile range, or above the third quartile plus twice the interquartile range, were considered outliers and were excluded from analyses.

We calculated exact percentiles, means and standard deviations of birthweight by sex for each gestational age between 20 and 44 weeks. Percentiles were tabulated and plotted by sex for each gestational age. Results for the fifth and 95th percentiles (and more extreme) are presented only for gestational ages with a minimum of 100 births, consistent with the previously published Australian percentiles. Mean birthweight was calculated by year and sex to examine any change over time.

We included births from all mothers, including mothers not born in Australia. Births were not classified by Indigenous status because of ethical restrictions on the use of the perinatal data collection.

All analyses were performed using SAS 9.2 for Windows (SAS Institute Inc, Cary, NC, USA).

The study was approved by the AIHW Ethics Committee (EC341). Approval for use of data was provided by all states and territories.

Results

Between 1998 and 2007, there were 2539 237 live singleton births recorded (Box 1). Of these infants, 5.9% were born preterm (birth before 37 completed weeks of gestation) while 4.8% were low birthweight (< 2500 g) and 0.8% very low birthweight (< 1500 g) (Box 1). We excluded from analysis 1610 births (0.1%) for which one or more of the key variables — sex, birthweight and gestational age — was missing; among these were 12 with gestational age more than 44 weeks and 146 with sex recorded as indeterminate.

Of the 2537 627 live singleton births with gestational age between 20 and 44 weeks and available data on sex and birthweight, 8986 (0.4%) were removed as outliers, with 7599 (0.3%) being above the higher Tukey limit, and 1387 (0.1%) being below the lower Tukey limit. Percentiles were calculated for a total of 2528 641 births (1300 273 males and 1228 368 females).

Box 2 shows birthweight percentiles by gestational age for male and female infants, and exact birthweight percentiles are listed in Box 3 and Box 4. Median birthweights were lower for female than male infants at all gestational ages apart from 44 weeks. The mean birthweight remained stable between 1998 and 2007 for both male and female infants (Box 5).

Discussion

The birthweight percentiles presented are based on 10 years of high-quality population data that have been shown to be accurate and complete.18 Our percentiles provide a reference for babies born in Australia and update those published in 1999 to better reflect the characteristics of Australian mothers. The data are sufficient in number to provide reliable percentiles for babies of early gestational age. Our approach to excluding implausible birthweights has been used in constructing birthweight percentiles in other countries.4,19 The high quality of Australian birthweight data is demonstrated by the fact that the percentiles from the remaining observations resulted in curves that did not need smoothing.

Changes such as the increase in maternal age, obesity and use of assisted reproductive technology7 have resulted in small increases in the gestational age- and sex-specific birthweights. Comparing term babies of the same gestational age, the median birthweight is between 0 and 25 g higher for male infants, and between 5 g and 45 g higher for female infants, than 10 years ago. Similar increases in 90th and most 10th percentiles for boys and girls were also observed. While these increases may seem small, at a population level they have a large impact. A mean increase in birthweight of 23 g between 1990 and 2005 for male babies in NSW translated into an 18% increase in those identified as large for gestational age. For female babies, an increase of 25 g translated into a 21% increase in those identified as large for gestational age.9 Increases in age-specific 10th and 90th percentiles observed from current data will therefore increase the rate of small for gestational age and decrease the rate of large for gestational age for term births, compared with using the previous percentiles.

It is noteworthy that the mean birthweights are relatively stable over time, with a maximum variation over a decade of 13 g for male infants, and 6 g for female infants, in spite of changes in maternal characteristics associated with birthweight.9 This apparent contradiction can be explained by the fact that the mean birthweights are not adjusted for gestational age. For example, while rates of smoking during pregnancy have decreased, there has been a reduction in gestational age (with preterm births increasing from 6.8% of all births in 199120 to 7.4% in 20077). Hence, the expected increase in average birthweight as a result of reduced maternal smoking may have been offset by decreased birthweights from preterm births. It is difficult to assess how other changes in maternal characteristics act together to explain the stability in mean birthweights, highlighting the need for birthweight percentiles to be presented separately by sex and gestational age.

In contrast to the birthweight percentiles published using 1991–1994 calendar year data,5 we calculated percentiles for all singleton births in Australia. Therefore, our percentiles may not be directly comparable with the earlier percentiles derived from non-Indigenous singleton births to Australian-born mothers.

Customised birthweight percentiles have been recommended,21 but their usefulness has been debated.22 There are two recognised features of customised percentiles: first, they use regression-based coefficients adjusted for maternal factors; and, second, they use estimated fetal weight. It is primarily the estimate of fetal weight that contributes to improved prediction of adverse perinatal outcomes, rather than adjustment for maternal characteristics.22 However, fetal (in-utero) weights are not routinely assessed in current practice. Furthermore, the proposition that identifying small for gestational age from customised percentiles, rather than from population reference curves, better predicts adverse outcomes is based on inconsistent evidence.23,24 A simulation study demonstrated that the use of customised percentiles did not improve the identification of infants with intrauterine growth restriction, and concluded that, for the customisation to be useful, the factors used in the customisation model would need to explain an unrealistically high amount of variability in actual birthweight.22 Whether differing points of view and fine areas of disagreement on customised and conventional birthweight percentiles for gestational age have important practical research or clinical implications is questionable.25

Australian birthweight percentiles continue to be used in clinical practice and research.15 The percentiles presented here have a role as a basis for identifying high-risk babies and are recognisable, practical and easily incorporated into charts and presentations. These percentiles provide an up-to-date reference for clinicians and researchers.

1 Maternal and infant characteristics — all live singleton births, Australia, 1998–2007

Characteristic

Number (%)


Total

2539 237

Sex of infant


Male

1305 356 (51.4%)

Female

1233 284 (48.6%)

Indeterminate

146

Not stated

451

Birthweight (g)


< 1500

19693 (0.8%)

1500–2499

102259 (4.0%)

2500–4499

2368 899 (93.3%)

= 4500

47672 (1.9%)

Not stated

714 (< 0.1%)

Gestational age (weeks)


20–31

22911 (0.9%)

32–36

126701 (5.0%)

37–41

2349 889 (92.5%)

42–44

39340 (1.5%)

> 44

12 (< 0.1%)

Not stated

384 (< 0.1%)

Maternal age (years)


< 20

119749 (4.7%)

20–24

387486 (15.3%)

25–29

735480 (29.0%)

30–34

821946 (32.4%)

35–39

398315 (15.7%)

> 40

75768 (3.0%)

Not stated

493 (< 0.1%)

Birth order

1st births

1047 055 (41.2%)

2nd or greater

1489 629 (58.7%)

Not stated

2553 (0.1%)

State

New South Wales

851912 (33.5%)

Victoria

621556 (24.5%)

Queensland

498582 (19.6%)

Western Australia

252444 (9.9%)

South Australia

175635 (6.9%)

Tasmania

56632 (2.2%)

Australia Capital Territory

47026 (1.9%)

Northern Territory

35450 (1.4%)

2 Birthweight percentiles for live singleton infants born to Australian women

3 Birthweight percentiles for live singleton male infants, Australia, 1998–2007

Gestational age (weeks)

Number of births

Mean (SD) birthweight (g)

Birthweight percentile (g)

1st

3rd

5th

10th

25th

50th

75th

90th

95th

97th

99th


20

230

349 (60)

210

248

254

273

310

340

390

430

450

470

500

21

335

418 (66)

270

290

300

335

375

420

460

500

540

542

575

22

401

505 (76)

350

370

390

410

460

500

554

600

630

650

690

23

395

595 (82)

390

450

470

500

540

588

650

700

730

756

800

24

640

681 (105)

426

470

500

550

618

684

750

810

850

875

970

25

715

783 (131)

440

505

530

620

700

785

865

944

995

1030

1100

26

937

894 (152)

500

576

621

680

802

900

996

1078

1130

1155

1210

27

1069

1016 (194)

510

605

660

752

904

1030

1138

1250

1320

1352

1440

28

1345

1146 (217)

591

680

735

844

1030

1165

1295

1395

1470

1522

1640

29

1524

1301 (252)

662

782

860

964

1150

1311

1463

1620

1700

1757

1860

30

2105

1474 (283)

774

900

984

1091

1300

1498

1650

1800

1920

1980

2182

31

2576

1666 (304)

915

1055

1126

1270

1480

1680

1855

2028

2142

2230

2435

32

3895

1867 (331)

1075

1214

1294

1430

1659

1880

2080

2270

2405

2503

2710

33

5599

2106 (371)

1200

1381

1473

1638

1880

2106

2340

2560

2710

2845

3070

34

9824

2340 (385)

1400

1580

1690

1860

2100

2340

2580

2810

2990

3120

3343

35

16054

2585 (408)

1600

1795

1920

2080

2330

2578

2835

3095

3275

3410

3665

36

32747

2826 (428)

1805

2015

2120

2295

2550

2820

3095

3360

3550

3690

3930

37

73986

3093 (449)

2050

2265

2372

2540

2800

3080

3378

3670

3865

3990

4235

38

230003

3344 (439)

2340

2540

2640

2800

3050

3330

3625

3910

4090

4215

4445

39

293109

3486 (430)

2510

2700

2800

2950

3195

3470

3765

4040

4220

4335

4560

40

409976

3632 (434)

2650

2840

2940

3090

3340

3620

3915

4195

4370

4490

4708

41

192154

3769 (438)

2780

2970

3070

3220

3470

3755

4060

4340

4515

4630

4850

42

19804

3832 (462)

2760

2980

3095

3250

3520

3820

4130

4430

4615

4740

4970

43

797

3761 (540)

2615

2785

2935

3085

3380

3750

4100

4470

4670

4825

5180

44

53

3715 (563)

3110

3300

3620

4070

4415

4 Birthweight percentiles for live singleton female infants, Australia, 1998–2007

Gestational age (weeks)

Number of births

Mean (SD) birthweight (g)

Birthweight percentile (g)

1st

3rd

5th

10th

25th

50th

75th

90th

95th

97th

99th


20

197

333 (65)

190

210

230

265

290

320

374

410

450

490

525

21

256

386 (69)

210

250

270

300

340

390

433

470

510

515

530

22

333

474 (72)

260

325

355

400

425

480

520

560

589

610

620

23

376

558 (89)

320

375

400

445

506

560

615

660

700

725

800

24

528

637 (95)

380

430

480

520

580

641

700

754

793

815

860

25

599

730 (128)

410

470

498

559

645

740

817

884

940

975

992

26

809

825 (166)

428

490

520

594

717

840

940

1026

1072

1106

1186

27

879

949 (188)

500

568

598

675

840

965

1077

1175

1240

1280

1390

28

1136

1073 (230)

495

622

675

764

928

1090

1230

1347

1410

1470

1610

29

1188

1215 (252)

572

712

790

870

1055

1240

1380

1494

1595

1680

1840

30

1656

1394 (277)

725

870

918

1030

1220

1400

1571

1715

1840

1920

2130

31

2052

1582 (302)

880

1000

1060

1190

1385

1590

1780

1948

2065

2146

2338

32

3119

1772 (322)

970

1140

1230

1348

1570

1780

1970

2170

2290

2400

2620

33

4421

2014 (356)

1180

1330

1424

1560

1790

2011

2235

2450

2616

2746

2970

34

8108

2242 (375)

1331

1525

1615

1764

2005

2240

2470

2705

2870

2995

3220

35

13104

2486 (403)

1525

1710

1820

1980

2230

2480

2735

2995

3175

3300

3516

36

28386

2720 (420)

1750

1940

2040

2198

2445

2710

2980

3250

3450

3575

3810

37

66928

2979 (439)

1970

2175

2275

2430

2690

2965

3255

3545

3735

3865

4100

38

214002

3215 (425)

2256

2440

2540

2690

2930

3200

3490

3770

3945

4062

4290

39

282046

3351 (415)

2420

2600

2690

2830

3070

3340

3620

3890

4060

4175

4390

40

398257

3493 (416)

2566

2740

2830

2975

3210

3480

3765

4030

4200

4316

4525

41

181434

3619 (424)

2680

2855

2945

3090

3330

3605

3900

4170

4340

4455

4670

42

17701

3665 (445)

2670

2850

2950

3110

3360

3650

3955

4240

4420

4545

4760

43

801

3579 (463)

2660

2800

2865

3010

3240

3560

3880

4210

4385

4560

4760

44

52

3705 (523)

3070

3403

3695

3965

4230

5 Mean birthweight (g) for live singleton infants by sex, Australia, 1998–2007

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007


Male

3462

3464

3470

3466

3461

3463

3463

3457

3461

3463

Female

3340

3335

3341

3341

3341

3339

3340

3338

3337

3339

Received 18 October 2011, accepted 20 May 2012

  • Timothy A Dobbins1
  • Elizabeth A Sullivan2
  • Christine L Roberts3
  • Judy M Simpson1

  • 1 Sydney School of Public Health, University of Sydney, Sydney, NSW.
  • 2 Perinatal and Reproductive Epidemiology Research Unit, University of New South Wales, Sydney, NSW.
  • 3 Clinical and Population Perinatal Health Research, Kolling Institute, University of Sydney, Sydney, NSW.


Acknowledgements: 

This article is based on data made available by the Australian Institute of Health and Welfare (AIHW). The analysis and views expressed are those of the authors, and do not necessarily reflect the views of the AIHW. Christine Roberts is supported by a National Health and Medical Research Council Senior Research Fellowship (#457078). We thank Zhuoyang Li, Research Officer, Perinatal and Reproductive Epidemiology Research Unit, School of Women’s and Children’s Health, University of New South Wales, for assistance with data processing.

Competing interests:

No relevant disclosures.

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