Continuous growth reference from 24th week of gestation to 24 months by gender

1471-2431-8-8 1471-2431 Research article Continuous growth reference from 24th week of gestation to 24 months by gender Niklasson Aimon aimon.niklasson@vgregion.se Albertsson-Wikland Kerstin kerstin.albertsson-wikland@pediat.gu.se

Göteborg Pediatric Growth Research Centre, Department of Pediatrics, the Institute for Clinical Sciences at the Sahlgrenska Academy, University of Gothenburg, Göteborg, Sweden

BMC Pediatrics 1471-2431 2008 8 1 8 http://www.biomedcentral.com/1471-2431/8/8 18307822 10.1186/1471-2431-8-8 13 3 2007 29 2 2008 29 2 2008 2008 Niklasson and Albertsson-Wikland; licensee 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.

Abstract

Background

Growth charts and child growth assessment have become prime global instruments in child health practice over the 30 years. An updated, continuous growth standard that bridges size at birth values with postnatal growth values can improve child growth screening and monitoring.

Methods

This novel growth chart was constructed from two sources of information. Size at birth (weight, length and head circumference) reference values were updated based on information of normal deliveries (i.e. singleton live births without severe congenital malformation, with healthy mothers and born vaginally) from the Swedish Medical Birth Registry, 1990–1999 (n = 810393). Weight was evaluated using logarithmic transformation as for postnatal weight. Standard deviations were estimated from data within the empirical mean ± 1.0 SD for each gestational week and gender. These values were smoothed by empirical curve-fitting together with values from our recently published postnatal growth reference including 3650 longitudinally followed children from birth to final height 9. Timescale and weight axes were made logarithmic in order to magnify the early time part of the graph.

Results

This study presents the first continuous gender specific growth chart from birth irrespective of gestational age at birth until 2 years of age for weight, length and head circumference. Birth weight at 40 weeks of gestation increased approximately 100 gram and length increased only 1 mm compared with earlier Swedish reference from 1977–81. The curve is now less S-shaped as compared with earlier curves and compared with 4 curves from other countries and with more constant variation over the whole range.

Conclusion

Our values picture the unrestricted pattern of growth improving the detection of a deviating growth pattern, when the growth of an individual infant is plotted on the charts. Especially for very preterm infants age corrected growth can be more easily evaluated although it must be recognized that the early comparison is with what is estimated as normal growth in uterus. The reference values are useful in child health care systems for population screening, but also in research or in the clinic for evaluating various growth promoting interventions – either nutritional, surgical or therapeutic – that might affect a child in early life.

Background

Growth charts and child growth assessment have become prime global instruments in child health practice over the past three decades 1. Many cross-sectional growth reference standards have been published, for both postnatal growth as well as at birth. However, so far there is no continuous growth standard bridging size at birth values with postnatal growth values, enabling growth of an individual child to be evaluated by means of one single growth chart, rather than two separate ones. There are many reasons why such a reference standard does not exist. One explanation is the lack of nationally- representative data for one or other of the two age periods. Another reason is that most national birth growth references – including the most recent – only include birth weight, but not length or head circumference 2345. Largo et al. have presented both intra-uterine and extra-uterine growth reference values for weight, length and head circumference based on two sets of series in the same chart 6; but the extra-uterine values were restricted to 8 weeks of age, and there was no attempt to join the two set of references into one single continuous smooth reference. A meta analysis of mean values in published growth reference studies 1980–2002 has been published, still only up to 10 weeks after full term 7 highlighting the clinical demand, not least for the neonatal units, for gender specific growth charts bridging the pre-postnatal growth period during which most catch up growth occur.

The latest updated Swedish national growth reference at birth was published in 1991, derived from the SMBR and based on the total Swedish cohort of infants born from 1977–1981 (n = 475 588), 8. A "healthy population" (79%) was extracted and weekly (28–42 weeks) grouped data for length, weight and head circumference were presented for boys and girls, respectively. The latest updated postnatal Swedish national growth reference of height, weight and length – from birth to 18 years of age – was introduced in 2000, derived from a longitudinal study of 3 650 healthy children born in Göteborg, Sweden in 1974 9.

The aim of the present study was to update the size at birth reference values for the total Swedish national birth cohort born between 1990 and 1999, not only for weeks 28–42 of gestation, but also for weeks 24 to 27, and to bridge these values by mathematical functions with the recently published postnatal growth reference values – for the first time producing continuous, smoothed and gender specific growth reference values from 24^thweek of gestation to 24 months of age.

Methods

The Swedish birth register

The study was based on data from the SMBR, 1990 to 1999, compiled from records through standardized sets of questionnaire completed at all antenatal care clinics and delivery units in Sweden, as well as during paediatric examinations of newborn infants. It records virtually all (98–99%) births in Sweden and the quality of the data was investigated and considered satisfactory 10. The records are linked to the Registry of Population, which includes the Death Registry.

Postnatal longitudinal growth study

The study material and methods have been described in detail elsewhere 911. Briefly, the study population comprised 5 111 pupils in the final grade of school in 1992 in Göteborg, Sweden. Of these, 174 pupils were not willing to participate, and another 449 failed to attend the investigation. Of the remaining cohort of 4 488, 76.8% were born in 1974, 16.8% in 1973, 3.6% in 1975 and 3.0% before 1973. In 1992, a special study team visited the schools. All participating children completed a health questionnaire as their weight and height were measured in a standard way, using a calibrated Harpenden stadiometer.

All health records for the children from birth until the final grade were obtained from Child Health Care Units and schools – an average of 14.4 measurements for each child. Information on parental height and weight was also obtained. Information from birth and the perinatal period was provided by the SMBR of the Swedish National Board for Health and Welfare. The group included only healthy children for whom there was information on gestational age and size at birth, and who were born at full term (37–43 weeks). A total of 529 children were excluded due to lack of birth data; half of them were not born in Sweden, 160 due to prematurity or post maturity, and another 141 due to chronic disease or medical treatment. The number of healthy full-term children who remained for the analysis, with complete longitudinal growth data for height, weight and head circumference from birth to final height, was 3 650 (1 801 girls and 1 849 boys).

The postnatal growth reference values used in this work – e.g. mean and SD values for height and weight from birth to final height – and head circumference from birth until 4 years of age have already been published elsewhere 9.

Statistical analysis

Body size at birth

It is a well known phenomenon that the standard deviation (SD) for all sizes at birth follows a u-shaped pattern over the gestational ages, with the highest values at week 24–30 23458. This phenomenon may be caused by a larger proportion of outlying observations in the lower and upper tail of the distribution for a certain body measurement in lower gestational ages than in higher ages, rather than reflecting a biological process 45. One reason for this is a wrongly determined length of gestation; and such an error has a larger impact on babies born prematurely than for term babies, due to the decelerating growth rate by age. Another reason may be caused by the fact that it is more difficult to take an accurate body measurement in premature babies than with full-term babies. There may also be an inflation of the SD values because each gestational age includes babies with 7 days variation in age; the latter will in that case be true for all gestational ages. We also made an adjustment for the range of ages within a gestational age, i.e. 7 days. The impact of this adjustment was minor, giving about a 1% reduction in the SD values for the size at birth measures.

First, we deleted the information for deliveries not regarded as normal from the analysis, in order to give size at birth reference values based on a "healthy" population. For this reason we excluded deliveries with stillbirths, severe congenital malformations, multiple births, maternal diseases (urine infection, kidney disease, diabetes, epilepsy, asthma, ulcerative colities and systemic lupus erythematosis) and records with missing gender and missing gestational age, and babies delivered by caesarean section with possible over- representation of growth retarded infants.

Second, we deleted all biologically unrealistic height, weight and head circumference values, e.g. all values lying outside mean ± 6.0 SD.

Based on the "healthy" sample, we estimated the mean and SD in three different ways: (a) based on the whole sample, (b) after adjusting for the variation in age, i.e. days within a gestational week, with individual values interpolated to the age representing the mean age of the interval by using the mean functional value for the body measurement over the interval, and (c) by using the central portion of the distribution for the age adjusted values – a distribution "trimming" method used for samples including all babies lying within mean ± 0.5 SD, mean ± 0.6 SD, .... and mean ± 2.0 SD. The algorithm of the trimming method is provided in the Appendix. We also computed the distribution skewness and kurtosis values for each sample distribution to estimate if the sample was approximately normally distributed or not. The mathematical formulas are in the Appendix.

We adopted a logarithmic transformation to birth weight values, since this is the transformation used from birth to final height 9. It has previously been reported that the birth weight distribution of Swedish newborns is positively skewed and that a transformation should preferably be applied 8. The optimum transformation was found to be a square root transformation.

We also estimated the mean and SD for the three body measurements by pooling the observations for gestational weeks 24–26 and 27–29, respectively, due to the relatively low sample size in lower ages. The individual values were interpolated to the age representing the mean age of the interval by using the mean functional value for the body measurement over the interval.

Body size at birth to final height

The postnatal growth reference mean and SD values for height, weight and head circumference from birth to final height used in this study are the same as the one previously published 9.

Curve fitting of the mean and SD values

Both the mean and SD values were fitted from gestational week 24 to final height by means of empirical mathematical functions as smoothing procedures for the graphs, omitting values after 40 weeks of gestational age in the first data set.

Results

Sample selection

The Swedish Medical Birth Register includes information for 1 053 425 births between 1990 and 1999. We excluded all stillbirths (n = 3 666), severe congenital malformations (n = 38 042), multiple births (n = 29 607), missing gender (n = 226), missing gestational age (n = 2 213), maternal diseases (urine infection, kidney disease, diabetes, epilepsy, asthma, ulcerative colitis and systemic lupus erythematosis – in total n = 141 348) and babies delivered by caesarean section (n = 55721 in which growth retardation is overrepresented). We also deleted unrealistic body measurement values at birth, i.e. outside ± 6 SD from the mean (n = 865). The information for the remaining 810 393 deliveries was kept for the analysis; 415 110 boys and 395 283 girls. Finally there were some missing values among the 810 393 selected cases so the effective sample sizes are 801 641, 808 312 and 775 399 for birth length, birth weight and head circumference, respectively.

Some differences in the mean size at birth values between the previous Swedish reference 8 and the one presented here were observed, reflecting a secular change from 1977–1981 to 1991–1999. For boys born at 40 weeks of gestation, mean birth weight increased from 3640.8 g to 3735.3 g, mean birth length from 51.22 to 51.28 and mean BMI from 13.80 to 14.17. For girls born at 40 weeks of gestation, mean birth weight increased from 3493.9 g to 3603.7 g, mean birth length from 50.33 to 50.44 and mean BMI from 13.76 to 14.13. The increase in mean weight and mean BMI was 3% for both sexes, while mean length increased by 0.1–0.2%.

Obtaining reference values

Tables 1, 2, 3 include the mean and SD values for length, weight and head circumference between 24 to 42 weeks of gestational length based on the total sample with and without adjustment – linear interpolation – for the exact age, i.e. in days rather than in weeks. The age adjusted SD values were about 1% less than the SD values computed on the total sample without an age adjustment: birth weight – boys 1.03%, girls 1.19%, all 1.11%; birth length – boys 1.30%, girls 1.03%, all 1.16%; head circumference – boys 1.12%, girls 1.12%, all 1.12%. However, the skewness and kurtosis values for the two ways of computing the reference values were similar. Virtually all kurtosis values for all three measurements and for both sexes were between 0 and 1, or close to zero – as expected for a normal distribution for a gestational age above 34 weeks. However, virtually all kurtosis values were much higher for the younger age groups.

Table 1

Summary statistics for length at birth by gestational age (GA) (weeks) based on raw data and adjusted data from the total population, and on the selected sample, Sweden, 1990–1999. Note: GA = 24 corresponds to 24 weeks+0 days etc as in the graphs.

Total

Adjusted

Delta = 1.0

Mean

SKEW

KURT

Mean

SKEW

KURT

Mean

SKEW

KURT

Boys

32.19

1.69

0.24

-0.61

-0.56

1.77

0.26

-0.20

-0.71

1.88

0.14

0.05

33.78

1.88

0.00

0.51

-0.64

1.81

-0.06

0.23

-0.65

1.50

-0.30

0.41

34.77

2.31

-1.75

5.78

-1.39

2.22

-1.81

5.91

-1.19

1.76

-0.08

-0.09

24–26

33.94

2.24

-0.64

1.52

-0.96

2.02

-1.15

4.68

-0.88

1.68

-0.22

0.09

37.07

3.50

1.86

5.53

-0.52

3.46

2.06

6.51

-0.97

2.13

0.01

-0.05

38.08

2.73

-0.35

3.22

-0.97

2.70

-0.26

3.14

-0.86

1.91

-0.46

0.71

39.87

3.45

0.02

3.79

-0.60

3.45

0.04

3.82

-0.55

2.17

-0.11

0.09

27–29

38.46

3.44

0.55

2.74

-0.70

3.22

0.74

5.06

-0.76

2.07

-0.15

0.17

41.32

2.81

0.45

3.18

-0.49

2.78

0.49

3.11

-0.55

2.24

-0.16

0.18

42.38

2.70

-0.12

2.11

-0.65

2.69

-0.13

2.09

-0.57

2.32

-0.14

-0.02

43.85

2.56

-0.56

3.12

-0.36

2.54

-0.51

3.15

-0.30

2.18

0.02

0.04

45.07

2.29

-0.43

2.43

-0.27

2.26

-0.43

2.48

-0.21

2.11

-0.09

0.02

46.21

2.19

-0.34

1.62

-0.20

2.17

-0.37

1.85

-0.15

1.96

-0.06

0.05

47.07

2.10

-0.29

1.53

-0.36

2.08

-0.28

1.52

-0.31

1.94

-0.07

0.03

47.96

2.02

-0.16

0.62

-0.45

2.00

-0.16

0.62

-0.41

1.93

-0.03

0.08

48.90

2.01

-0.18

0.81

-0.48

1.99

-0.19

0.84

-0.44

1.89

-0.12

0.05

49.80

1.92

-0.09

0.74

-0.49

1.91

-0.09

0.76

-0.48

1.80

-0.05

0.19

50.58

1.85

-0.05

0.70

-0.57

1.84

-0.05

0.71

-0.57

1.74

0.05

0.04

51.28

1.84

0.04

0.41

-0.68

1.83

0.04

0.41

-0.70

1.79

0.09

-0.06

51.89

1.87

0.02

0.41

-0.87

1.86

0.02

0.42

-0.84

1.88

0.00

-0.05

52.40

1.91

0.00

0.35

-1.00

1.90

-0.03

0.65

-1.00

1.92

0.13

0.18

Girls

32.45

3.45

3.62

18.24

-0.53

3.57

3.79

19.48

-1.02

1.95

-0.06

-0.34

33.88

3.99

3.23

12.96

-0.55

4.06

3.33

13.42

-1.23

1.91

0.03

0.09

35.15

2.53

0.98

6.88

-0.86

2.49

0.94

7.66

-0.98

1.82

0.09

0.14

24–26

34.11

3.42

2.32

9.92

-0.69

3.29

3.18

16.26

-1.03

1.85

0.07

-0.13

36.60

4.16

1.73

4.74

-0.73

4.13

1.77

5.00

-1.11

2.57

-0.20

1.00

37.50

2.75

0.34

2.57

-1.34

2.69

0.27

2.51

-1.35

2.35

-0.18

0.08

39.57

2.55

0.88

5.35

-0.66

2.49

0.88

5.28

-0.68

2.03

-0.49

0.43

27–29

38.19

3.31

0.68

3.05

-0.89

3.03

1.35

6.31

-0.96

2.25

-0.22

0.12

40.80

2.73

0.68

3.97

-0.51

2.71

0.75

4.03

-0.56

1.99

-0.33

0.14

42.39

2.69

0.63

1.99

-0.16

2.66

0.66

2.26

-0.25

2.14

-0.14

-0.22

43.24

2.52

-0.31

3.80

-0.46

2.50

-0.30

3.93

-0.41

1.98

-0.14

0.41

44.53

2.26

-0.24

1.66

-0.25

2.26

-0.27

1.69

-0.21

2.08

-0.08

0.03

45.58

2.35

-0.34

2.60

-0.22

2.35

-0.31

2.67

-0.16

2.08

-0.05

46.48

2.11

-0.18

0.78

-0.31

2.09

-0.17

0.79

-0.28

2.00

0.00

-0.03

47.37

2.02

-0.15

0.45

-0.37

2.01

-0.14

0.42

-0.34

1.93

-0.07

-0.05

48.23

1.97

-0.04

0.73

-0.44

1.95

-0.03

0.73

-0.42

1.87

0.01

-0.01

49.03

1.89

-0.13

0.72

-0.52

1.88

-0.12

0.72

-0.49

1.81

-0.08

0.05

49.79

1.81

-0.07

0.75

-0.59

1.80

-0.07

0.76

-0.59

1.70

-0.08

0.15

50.44

1.81

-0.04

0.80

-0.71

1.80

-0.04

0.81

-0.72

1.72

0.08

0.07

51.01

1.83

0.03

0.54

-0.92

1.82

0.02

0.53

-0.84

1.77

0.08

0.35

51.44

1.85

0.10

0.41

-1.13

1.84

0.12

0.53

-1.15

1.82

0.22

0.04

Table 2

Summary statistics for log₁₀[weight(kg)] at birth by gestational age (GA) (weeks) based on raw data and adjusted data from the total population, and on the selected sample, Sweden, 1990–1999. Note: GA = 24 corresponds to 24 weeks+0 days etc as in the graphs.

Total

Adjusted

Delta = 1.0

Mean

SKEW

KURT

Mean

SKEW

KURT

Mean

SKEW

KURT

Boys

-0.16

0.14

-6.14

47.15

-0.029

0.144

-6.47

50.68

-0.013

0.060

0.18

0.26

-0.07

0.16

0.30

23.98

-0.013

0.159

0.21

24.03

-0.022

0.053

-0.42

0.45

-0.03

0.08

-0.25

5.05

-0.037

0.078

-0.30

5.12

-0.031

0.056

0.04

0.15

24–26

-0.07

0.14

-1.64

31.64

-0.026

0.128

-1.70

38.18

-0.024

0.055

-0.16

0.03

0.04

0.10

1.76

9.23

-0.028

0.103

1.93

10.93

-0.029

0.058

-0.07

0.29

0.09

0.08

-1.41

6.08

-0.031

0.081

-1.31

6.34

-0.026

0.058

-0.11

0.24

0.14

0.10

-0.07

4.13

-0.038

0.100

-0.06

4.25

-0.031

0.066

-0.41

0.31

27–29

0.09

0.11

0.25

3.72

-0.033

0.095

0.47

7.72

-0.028

0.060

-0.21

0.25

0.19

0.10

0.11

3.63

-0.036

0.094

0.20

3.90

-0.033

0.064

-0.06

-0.11

0.24

0.09

-0.61

4.16

-0.036

0.088

-0.64

4.16

-0.030

0.063

-0.32

0.38

0.30

0.08

-0.28

3.58

-0.023

0.080

-0.25

3.73

-0.021

0.062

-0.12

0.00

0.35

0.07

-0.44

4.25

-0.015

0.074

-0.39

4.41

-0.013

0.057

-0.19

0.19

0.39

0.07

-0.52

2.94

-0.010

0.070

-0.54

3.23

-0.008

0.057

0.00

0.10

0.43

0.07

-0.58

2.33

-0.008

0.065

-0.58

2.35

-0.005

0.056

-0.10

-0.01

0.46

0.06

-0.42

1.29

-0.006

0.063

-0.43

1.33

-0.004

0.057

-0.10

0.02

0.50

0.06

-0.37

1.16

-0.004

0.060

-0.37

1.16

-0.002

0.054

-0.05

0.04

0.53

0.06

-0.30

0.83

-0.004

0.056

-0.29

0.84

-0.002

0.053

-0.07

0.03

0.55

0.05

-0.27

0.68

-0.006

0.053

-0.26

0.68

-0.005

0.051

-0.05

0.02

0.57

0.05

-0.21

0.45

-0.011

0.052

-0.21

0.46

-0.010

0.050

-0.04

0.02

0.58

0.05

-0.24

0.42

-0.019

0.052

-0.25

0.44

-0.014

0.051

-0.06

0.02

0.59

0.05

-0.27

0.39

-0.024

0.052

-0.22

0.40

-0.023

0.052

-0.06

0.00

Girls

-0.15

0.10

4.20

28.58

-0.009

0.106

4.38

30.13

-0.016

0.063

-0.22

-0.06

-0.08

0.12

3.02

15.07

-0.006

0.122

3.20

16.26

-0.017

0.057

0.16

0.77

-0.05

0.08

-1.10

2.86

-0.044

0.079

-1.31

3.37

-0.036

0.055

-0.44

0.91

24–26

-0.09

0.11

1.92

11.18

-0.022

0.104

2.87

20.82

-0.024

0.057

0.02

0.20

0.02

0.14

1.75

5.95

-0.026

0.138

1.88

6.73

-0.035

0.073

-0.36

0.23

0.05

0.10

-0.67

1.53

-0.065

0.095

-0.73

2.21

-0.055

0.077

-0.32

0.55

0.13

0.08

-0.84

4.46

-0.040

0.083

-0.91

5.12

-0.034

0.064

-0.12

-0.04

27–29

0.07

0.11

0.23

3.05

-0.045

0.105

0.87

7.90

-0.040

0.070