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. 2015 Oct 8;15:148. doi: 10.1186/s12887-015-0464-y

Australian national birthweight percentiles by sex and gestational age for twins, 2001–2010

Zhuoyang Li 1,2, Mark P Umstad 3,4, Lisa Hilder 2, Fenglian Xu 1, Elizabeth A Sullivan 1,2,✉
PMCID: PMC4599725  PMID: 26450410

Abstract

Background

Birthweight remains one of the strongest predictors of perinatal mortality and disability. Birthweight percentiles form a reference that allows the detection of neonates at higher risk of neonatal and postneonatal morbidity. The aim of the study is to present updated national birthweight percentiles by gestational age for male and female twins born in Australia.

Methods

Population data were extracted from the Australian National Perinatal Data Collection for twins born in Australia between 2001 and 2010. A total of 43,833 women gave birth to 87,666 twins in Australia which were included in the study analysis. Implausible birthweights were excluded using Tukey’s methodology based on the interquartile range. Univariate analysis was used to examine the birthweight percentiles for liveborn twins born between 20 and 42 weeks gestation.

Results

Birthweight percentiles by gestational age were calculated for 85,925 live births (43,153 males and 42,706 females). Of these infants, 53.6 % were born preterm (birth before 37 completed weeks of gestation) while 50.2 % were low birthweight (<2500 g) and 8.7 % were very low birthweight (<1500 g). The mean birthweight decreased from 2462 g in 2001 to 2440 g in 2010 for male twins, compared with 2485 g in 1991–94. For female twins, the mean birthweight decreased from 2375 g in 2001 to 2338 g in 2010, compared with 2382 g in 1991–94.

Conclusions

The birthweight percentiles provide clinicians and researchers with up-to-date population norms of birthweight percentiles for twins in Australia.

Keywords: Twins, Birth weight, Gestational age, Small for gestational age

Background

Birthweight remains one of the strongest predictors of perinatal mortality and disability [1, 2]. Birthweight percentiles form a reference that incorporates weight and gestational age of infants at birth and are used as an adjunct for detecting neonates with suspected intra-uterine growth impairment and those at higher risk of neonatal and postneonatal morbidity. Twin births account for about 3 % of all births in Australia but make a significantly greater contribution to perinatal morbidity and mortality than singleton births [3]. Australia’s first birthweight percentiles for twin births based on national population data were published in 1999 using live twins born during 1991–94 [4]. Marked socio-demographic changes in maternal characteristics and clinical practice have occurred during the period since this publication including increased maternal age, reduced smoking rate, and increased usage of assisted reproductive technology [2].

The aim of the study is to present updated national birthweight percentiles for all male and female liveborn twins born in Australia over the 10-year period between 2001 and 2010.

Methods

Population-based data on twins born in Australia between January 2001 and December 2010 were obtained from Australian Institute of Health and Welfare National Perinatal Data Collection (NPDC). The NPDC is a national collation of jurisdictional population-based cross sectional data collections of pregnancy, childbirth and perinatal outcomes. Information is included in the NPDC on both live births and stillbirths of at least 400 g birthweight or at least 20 weeks gestation.

Records with missing birthweight, infant sex or gestational age values were excluded from calculating the birthweight-by-gestation percentiles. In addition, records with implausible birthweight were identified using Tukey’s methodology [5] based on the interquartile range. Birthweights for each sex and gestational age combination that fell below the first quartile minus twice the interquartile range (lower Tukey limit) or above the third quartile plus twice the interquartile range (higher Tukey limit) were considered outliers and were excluded from the analyses.

Level of remoteness was based on the geographical location of the usual residence of the mother, and was classified into five groups: major cities of Australia, inner regional Australia, outer regional Australia, remote Australia and very remote Australia.

Univariate analysis was used to examine the birthweight distributions and to determine the interquartile range for each gestational age for twins born in Australia. After removing outliers exact percentiles of birthweight in grams were calculated for each gestational week between 20 and 42 weeks. Results for the 5th and 95th percentile are presented only for gestational ages with a minimum of 100 records and the 10th and 90th percentile are plotted only for gestational ages with a minimum of 50 records, to be consistent with previously published Australian birthweight percentiles [2, 4]. Student t-test was used to examine the mean birthweight difference between twins born in 1991–94 and 2010. General linear model was used to investigate the trends for mean birthweight for male and female twins born between 2001 and 2010. All analyses were carried out using SAS for Windows, version 9.3 (SAS Inc, Cary, NC).

Ethics approval for this study was granted by the Human Research Ethics Advisory Panel of the University of New South Wale, Australia (Reference number: 2013-7-07) and Australian Institute of Health and Welfare Ethics Committee (Reference number: EO 2013/2/17). As secondary data analysis of de-identified data set, additional consent from participants was not required. Approval for use of data was provided by all states and territories.

Results

Between 2001 and 2010, a total of 43,833 women gave birth to 87,666 twins in Australia. Table 1 presents the maternal demographic and obstetric characteristics of these women. 1741 (2.0 %) births (1737 stillbirths and 4 births with unknown vital status at birth) were not considered further.

Table 1.

Maternal characteristics of women who gave birth to twins, Australia, 2001-2010

Maternal characteristics Number & percentage
Total 43,833 (100.0 %)
Maternal age (years)
   < 20 873 (2.0 %)
  20-24 4076 (9.3 %)
  25-29 10,468 (23.9 %)
  30-34 15,944 (36.4 %)
  35-39 10,412 (23.8 %)
   > =40 2055 (4.7 %)
  Not stated 5 (0.0 %)
Parity
  Primiparas 17,971 (43.3 %)
  Multiparas 23,466 (56.6 %)
  Not stated 37 (0.1 %)
Country of birth
  Australia 31,682 (72.3 %)
  Overseas 11,944 (27.2 %)
  Not stated 207 (0.5 %)
Smoking during pregnancy
  Yes 4271 (13.9 %)
  No 25,968 (84.7 %)
  Not stated 420 (1.4 %)
Remoteness
  Major Cities 30,579 (69.8 %)
  Inner Regional 8136 (18.6 %)
  Outer Regional 3959 (9.0 %)
  Remote 738 (1.7 %)
  Very remote 367 (0.8 %)
  Not stated 17 (0.0 %)
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Among the 85,925 live births (43,153 males and 42,706 females), 53.6 % were born preterm (birth before 37 completed weeks of gestation) while 50.2 % were low birthweight (<2500 g) and 8.7 % were very low birthweight (<1500 g) (Table 2). More than half of liveborn twins were admitted to a special care nursery or neonatal intensive care unit (58.6 %) or required some type of active resuscitation measures (54.0 %) (Table 2). The median length of stay in hospital for twins was 6 days (interquartile range: 5 – 13 days). The 121 (0.1 %) records missing one or more of the key variables (sex, birthweight and gestational age), and 134 (0.2 %) lower Tukey limit and 207 (0.2 %) higher Tukey limit outliers were excluded. Percentiles were calculated for the remaining 85,436 infants.

Table 2.

Live twin births, Australia, 2001-2010

Infant characteristics Number & percentage
Total 85,925 (100.0 %)
Sex
  Male 43,153 (50.2 %)
  Female 42,706 (49.7 %)
  Not stated 66 (0.1 %)
Birthweight (g)
   < 1500 7461 (8.7 %)
  1500-2499 35,689 (41.5 %)
  2500-2999 30,403 (35.4 %)
  3000-3999 12,262 (14.3 %)
   > =4000 68 (0.1 %)
  Not stated 42 (0.0 %)
Gestational age (weeks)
  20-27 2845 (3.3 %)
  28-31 5501 (6.4 %)
  32-36 37,745 (43.9 %)
  37-41 39,792 (46.3 %)
   > =42 26 (0.0 %)
  Not stated 16 (0.0 %)
Presentation
  Vertex 58,727 (68.3 %)
  Breech 23,605 (27.5 %)
  Other 2566 (3.0 %)
  Not stated 1027 (1.2 %)
Apgar score at 5 minutes
  0-3 1124 (1.3 %)
  4-6 1810 (2.1 %)
  7-10 82,849 (96.4 %)
  Not stated 142 (0.2 %)
Resuscitation
  Yes 46,427 (54.0 %)
  No 34,581 (40.2 %)
  Not stated 4917 (5.7 %)
Admission to NICU
  Yes 47,674 (58.6 %)
  No 33,370 (41.0 %)
  Not stated 278 (0.3 %)
Length of stay
  Less than 1 day 2645 (3.1 %)
  1 day 1502 (1.7 %)
  2 day 2582 (3.0 %)
  3 day 5102 (5.9 %)
  4 day 9289 (10.8 %)
  5 day 12,231 (14.2 %)
  6 day 9704 (11.3 %)
  7-13 days 21,520 (25.0 %)
  14-20 days 9138 (10.6 %)
  21-27 days 4858 (5.7 %)
  28 or more days 7204 (8.4 %)
  Not stated 150 (0.2 %)
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Figure 1 shows birthweight percentiles by gestational age for liveborn twins by infant sex; exact birthweight percentiles are shown in Table 3 and Table 4. The mean birthweight slightly decreased from 2462 g in 2001 to 2440 g in 2010 for male twins (p = 0.49). For female twins, the mean birthweight significantly decreased from 2375 g in 2001 to 2338 g in 2010 (p < 0.001) (Fig. 2). Compared with twins born in Australian 1991–94, the mean birthweight was significantly lower for twins born in 2010 for both male (2485 g versus 2440 g, p < 0.001) and female (2382 g versus 2338 g, p < 0.001).

Fig. 1.

Fig. 1

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Birthweight percentiles for liveborn twins, by sex, Australia, 2001–2010

Table 3.

Birthweight percentiles for male liveborn twins, Australia, 2001-2010

Gestation (weeks) No. of births Mean (g) Standard deviation Birthweight percentile (g)
p3 p5 p10 p25 p50 p75 p90 p95 p97
20 42 338 50 . . . 300 340 369 . . .
21 90 402 73 . . 320 355 400 450 495 . .
22 131 495 72 320 350 400 450 510 540 580 600 620
23 166 560 77 400 440 470 505 560 610 650 695 718
24 216 666 96 477 490 525 617 673 724 784 820 840
25 234 760 114 510 540 610 695 760 840 900 940 968
26 297 883 161 555 610 686 775 880 986 1080 1156 1205
27 323 1010 161 616 716 804 912 1019 1116 1200 1270 1300
28 478 1153 201 720 780 890 1030 1160 1290 1410 1450 1505
29 563 1297 219 757 875 1000 1180 1327 1450 1545 1625 1660
30 720 1453 246 930 991 1128 1314 1477 1600 1755 1850 1928
31 1059 1628 255 1120 1172 1285 1470 1632 1790 1942 2060 2120
32 1638 1784 292 1180 1275 1400 1596 1799 1972 2155 2245 2316
33 2040 1999 304 1410 1469 1590 1815 2010 2200 2370 2486 2555
34 3408 2179 328 1530 1618 1760 1970 2180 2400 2600 2704 2780
35 4503 2382 340 1755 1830 1950 2155 2380 2600 2810 2950 3040
36 7319 2589 359 1910 1995 2135 2350 2595 2820 3044 3185 3280
37 10665 2768 355 2100 2190 2315 2530 2765 3000 3215 3356 3470
38 7601 2901 357 2240 2320 2450 2665 2894 3130 3365 3510 3620
39 1129 2982 364 2260 2415 2538 2730 2980 3220 3460 3600 3695
40 308 3031 427 2275 2330 2470 2743 3040 3275 3600 3790 3960
41 25 3199 558 . . . 2805 3015 3500 . . .
42 8 2969 459 . . . 2500 2980 3400 . . .
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Table 4.

Birthweight percentiles for female liveborn twins, Australia, 2001-2010

Gestation (weeks) No. of births Mean (g) Standard deviation Birthweight percentile (g)
p3 p5 p10 p25 p50 p75 p90 p95 p97
20 49 299 57 . . . 260 296 330 . . .
21 94 374 58 . . 300 340 370 415 450 . .
22 107 459 75 300 330 350 420 460 508 545 570 580
23 143 517 83 325 350 405 470 520 570 620 640 650
24 176 635 87 425 480 530 589 638 700 731 751 780
25 167 728 113 520 547 580 656 730 800 878 907 920
26 241 845 141 525 580 660 760 864 930 1000 1052 1090
27 308 951 186 550 598 710 840 983 1060 1180 1240 1290
28 384 1090 169 745 795 870 991 1112 1200 1280 1345 1365
29 542 1219 211 750 808 930 1101 1235 1360 1465 1536 1601
30 697 1355 244 850 896 1010 1210 1375 1530 1655 1725 1760
31 990 1551 239 1030 1105 1227 1415 1568 1700 1833 1930 1990
32 1618 1713 262 1182 1248 1365 1550 1730 1880 2020 2140 2220
33 2037 1895 288 1320 1400 1530 1713 1900 2086 2250 2360 2430
34 3322 2085 308 1482 1555 1685 1885 2090 2292 2480 2570 2635
35 4558 2267 324 1640 1715 1850 2060 2270 2480 2670 2790 2875
36 7145 2464 339 1825 1905 2035 2240 2460 2680 2900 3030 3130
37 10695 2656 340 2020 2100 2225 2430 2650 2880 3090 3230 3326
38 7720 2774 339 2145 2230 2349 2545 2765 2990 3215 3350 3435
39 1154 2884 373 2185 2280 2420 2630 2875 3140 3360 3500 3570
40 320 2958 438 2110 2180 2368 2668 3000 3240 3518 3653 3800
41 22 2976 322 . . . 2795 2933 3220 . . .
42 11 3001 497 . . . 2540 2910 3440 . . .
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Fig. 2.

Fig. 2

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Mean birthweight of liveborn twins, by sex, Australia, 2001–2010

Discussion

We developed the contemporary population-based birthweight percentile charts to provide an up-to-date reference for twins born in Australia.

Growth in twin pregnancies slows progressively from around 32 weeks until term [6]. When comparing the dataset in this study with a singleton population from a similar time period [2] this phenomenon is confirmed. The median gestation-specific birthweight for twins were remarkably similar to those for singletons from a similar era until around 32 weeks gestation. The difference then becomes progressively more pronounced, reaching 640 g for males and 480 g for females at 40 weeks. It has been postulated that this slowing of growth late in a multiple pregnancy is a physiologically adaptive process that favours developmental maturity at the expense of fetal size [6]. However, there is a limit to this physiological adaptation and eventually the growth restriction becomes a pathological process. This is evidenced by studies consistently highlighting the risks of late fetal death in twins, particularly in relation to growth restriction [7–9].

Twin pregnancies have an increased risk of adverse outcome compared to single pregnancies. The increase in perinatal mortality and morbidity is primarily related to preterm delivery, complications of monochorionicity, and fetal growth restriction. Accurate population data is required to accurately diagnose growth restriction in twin pregnancies.

Secular trends for a decrease in the overall mean birthweight for liveborn twins were observed for both male and females in Australia in the study period. In contrast the mean birthweight for liveborn singletons in Australia from a similar time period has been relatively stable [2]. The observed decline in the mean birthweight for twins over time was partially explained by the downward shift in the distribution of gestational age. The mean gestational age was 35.4 weeks for twins in 2001, compared with 35.1 weeks in 2010 [10, 11]. Preterm birth occurred in 51.2 % of twins in 2001 and 56.7 % in 2010 [10, 11]. This decreased mean gestational age and increased pre-term births rate among twins might be attributed to improved intrapartum survival at earlier gestations, earlier obstetric interventions and increased usage of assisted reproductive technology [12, 13].

Population-based birthweight percentile charts for twins are scarce [14–17]. When comparing Australian birthweight percentile charts with others, significant differences in birth size are observed between populations. At 40 weeks gestation, the mean birthweight of Australian live born twins are markedly lower than Norwegian and Finnish counterparts [15, 17] but the median birthweight of Australian twins are heavier than Canadian and Japanese twins [14, 16]. Sankilampi et al. has stated that the differences in term birth size is largely associated with differences in genetic background rather than maternal nutrition or healthcare as this is comparable between these developed countries [17].

There is inherent appeal in the customisation of growth charts that incorporate maternal weight, height, ethnicity as well as plurality. The intent is to identify fetuses that are small as a consequence of growth restriction rather than constitutionally small for clinical decision-making. Several large observational studies have suggested that customised charts improve the identification of infants with intrauterine growth restriction compared with population based charts, although contrasting opinions exist [18–22]. Birthweight centiles provide a population reference describing fetal growth in the population rather than a standard for assessing individual growth [23]. They also provide the basis for characterising newborn size for longitudinal studies of childhood outcomes. The usefulness of customised birthweight percentiles has been debated. The value of plurality-specific centiles has been established but the literature remains divided on the benefits of customisation for other characteristics and the need for multiple reference charts [24]. Maternal height and weight have been included in the NPDC from 2010, but agreed national standards have not yet been implemented. Further studies are required to examine whether customised growth charts adjusted for maternal size and ethnicity contributes to improved prediction of adverse perinatal outcomes.

The provision of these updated birthweight percentile charts allows clinicians and researchers to re-evaluate the success of pregnancy management by determining the rate of detection of growth restriction. As previously described, growth restriction is the major cause of late fetal death in twin pregnancies and its accurate diagnosis a focal point of good obstetric care.

One limitation of this study is that birthweight percentile charts do not measure intrauterine growth but rather size at birth. The birthweight of babies born prematurely is likely to be influenced by the pathological process leading to preterm birth and therefore likely to differ from those remaining in utero until term [25, 26]. It has been argued that the preterm births should be assessed using estimated fetal weight rather than birthweight percentile charts as preterm neonates are disproportionately affected by the fetal growth restriction [25]. However, the accuracy of estimated fetal weight is limited by the ability of obtaining accurate measurements included within the computation of estimated fetal weight and the formula used for computation [26, 27]. To date, there has been no Australian chart published for sonographic standards for estimated fetal weight and the Australasian Society for Ultrasound in Medicine’s position is that ‘No formula for estimating fetal weight has achieved an accuracy which enables us to recommend its use’ [27, 28]. In such cases, the population-based birthweight percentile charts presented in this study provide a valuable reference for clinicians and researchers assessing the prognosis of twins in Australia.

Conclusions

This study presents the up-to-date national population-based birthweight percentile charts for male and female live born twins in Australia. These new charts provide a valuable reference for clinicians and researchers correctly identifying high-risk twins in Australia.

Acknowledgements

This research is based on data made available by the Australian Institute of Health and Welfare (AIHW). The authors acknowledge the AIHW for funding the NPDC and midwives and other health professionals for collecting data for the NPDC.

Abbreviations

AIHW

Australian Institute of Health and Welfare

NPDC

National Perinatal Data Collection

Footnotes

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

All authors participated in the study design and interpretation of data. ZL conducted the data analysis and drafted the manuscript. MPU participated in drafting the manuscript. LH participated in the data analysis and revised the manuscript critically. FX and EAS have revised the manuscript critically for important intellectual content. All authors read and approved the final manuscript.

Authors’ information

Not applicable.

Contributor Information

Zhuoyang Li, Email: zhuoyang.li@uts.edu.au.

Mark P. Umstad, Email: umstad@bigpond.net.au

Lisa Hilder, Email: l.hilder@unsw.edu.au.

Fenglian Xu, Email: Fenglian.Xu@uts.edu.au.

Elizabeth A. Sullivan, Phone: +61 2 9514 4833, Email: elizabeth.sullivan@uts.edu.au

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