A Longitudinal Study for Developing Growth Charts in a Middle Eastern Child Population Under Two Years Old and Comparison with the World Health Organization Reference Curves

Zahra Heidari¹, Awat Feizi^1,2,3*, Azimeh Maghzi Najafabadi¹ and Roya Kelishadi⁴

^*Correspondence: Awat Feizi, Cardiac Rehabilitation Research Centre, Cardiovascular Research Institute, Isfahan University of Medical Sciences, Iran, Email:

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Introduction

Growth is a sensitive indicator of the health of infants and children. The growth assessment plays a pivotal role in child health monitoring. Growth failure, nutritional disorders, chronic illness, and social deprivation can be detected early through growth monitoring [1,2]. Growth charts in the form of curves or tables allow the visual inspection of anthropometric parameters and are widely used to evaluate growth and health status of children and adolescents. They play an essential role in the detection and assessment of abnormalities in growth and development [1,3,4].

Although international growth charts for anthropometrics indices exist such as Centers for Disease Control and Prevention (CDC) Growth Charts and the World Health Organization (WHO) Child Growth Standards (WHO-CGS), national and local continents growth charts are unavailable in many countries; Although some countries had their own growth curves or have adopted the WHO growth charts, few countries did not adopt them and some countries such as united states of America partially adopted the new growth standards for age under 2 years and in some instance such as European Society for Pediatric Gastroenterology, Hepatology and Nutrition Committee on Nutrition directly recommends that the WHO reference charts should be adopted for 0-5 year-old children in Europe [5-7]. However, it should be noted that the World Health Organization (WHO) Growth Standards for children aged 0-5 years were established by data from six countries (USA, Norway, Brazil, Ghana, India, and Oman) with sampling from children living under the optimal environment for growth such as breastfed, nonsmoking mother and adequate access to healthcare [8-11]. Despite of the CDC and WHO growth reference charts are commonly used in some countries, their applicability are challenging due to the diversity in cultural, economic, social and environmental factors such as diet and health care services and genetically background all influence the growth of people. Accordingly, growth assessment in children can be improved by considering all above mentioned influential factors through establishing the local and national growth references [9-11]. Therefore, many countries, such as Norway, UK, China, Taiwan, Saudi Arabia, and Japan had established their own growth references [3].

There is no national reference charts in Iran for children aged under five; although, in recent years some local growth charts have been developed in Iran [12-17], but they did not evaluate and construct the reference charts for all common anthropometric indices and majority of them focused only on one measure or specific age group such as newborn and more important lack on considering the reliable protocol for evaluating the reference chart for children under 2 year particularly tracking the growth from birth to the end of 2 year old longitudinally. Here, we developed growth references charts for all three anthropometric measures i.e., head circumference-for-age, length-for-age, and weight-for-age based on a large sample representative local data for children living in Isfahan, the biggest city located in center and the third biggest city of Iran, aged 0-2 years and compared these references with those reported by WHO.

Materials and Methods

This population-based, longitudinal study was conducted on 1088 healthy children (542 boys and 546 girls) who were born between 2012 to 2015 in Isfahan city, central of Iran. Isfahan is the thirds city of Iran with 22,50,000 population based on last census at 2017. The current study samples were selected from urban health centers of Isfahan through stratified by gender and multistage cluster sampling, and the selected children were followed from birth until 24 months. Isfahan city has two main health centers i.e., the Isfahan focal health centers I and II, both supervised some local health centers. We adopted the WHO protocol for selecting our study sample from two main clusters i.e., focal center I and II and local health centers that were supervised [6,18]. Local health centers under supervision of two focal heal centers located in different geographic region of Isfahan city. We selected local centers based on considering geographic coverage of the city for getting representative of a sample and in each selected local health center, we provided a list of children that fulfilled the WHO criteria for constructing reference charts and finally we selected our proposed sample based on a systematic random sampling from the prepared list. Firstly, 1088 healthy children, that capture our inclusion criteria, were identified, and their information was recorded from their medical records. Among these children, 73 children were born in 2012, 695 in 2013, 209 in 2014, and 111 in 2015. In accordance with the WHO's instructions [6,18], inclusion criteria were as follows: Iranian citizenship, absence of significant morbidity or disease, lack of known health or environmental constraints to growth, appropriate socioeconomic status of the family, single and full-term birth, no maternal smoking before and after delivery, exclusive or predominant breastfeeding for at least 4 months, introduction of complementary foods by the age of 4-6 months, and continued partial breastfeeding up to at least 12 months. The confidentiality of all obtained information was administered carefully by two reliable experts in each health center and also researchers of the study. The design of the current study was approved by Ethics Committee of Isfahan University of Medical Sciences (Project Number: 193122).

Anthropometric measurements

At birth and during each follow-up time points i.e., 3, 6, 9, 12 and 18 months (for head circumference) and 24 months for (weigh and length), anthropometric measures (head circumference, length, and weight) of children were measured by trained health staff by using calibrated instruments under standard protocols [6,18]. Children's head circumference in centimeter (cm) was measured using a non-elastic tape with a precision of 0.1 cm. The length of children in cm was measured in a supine position without shoes by using a tape meter with a precision of 0.1 cm. Children’s weight in kilograms (kg) was measured with a precision of 10 g on an electronic scale that was placed on a flat ground and participants were motionless and wearing light clothing.

Statistical analysis

We used the Lambda-Mu-Sigma (LMS) method to develop new growth references for both boys and girls from birth to 24 months. The LMS method summarizes the distribution of the data by age and sex in terms of three parameters called lambda (L), mu (M) and sigma (S). The M parameter is the median by age, the S parameter is the coefficient of variation, and the L parameter expresses the skewness of the distribution in terms of the Box-Cox power needed to transform the data to near normality [6,18,19]. For each data set, curves of the values of these 3 parameters as a function of age are presented. In the LMS model, the smoothness of the L, M and S curves is characterized by the number of equivalent degrees of freedom (edf). In the current study, the selection of the degree of freedom (df) was guided by the deviance statistic, Q-test and worm plot, a tool consists of a number of detrended Q-Q plots, split according to age. A model that fits the data well is characterized by ‘ flat worms’ [6,18,19]. The use of calculated percentile values per se made it difficult to reflect the trend for growth due to fluctuations of age and height. Therefore, the curve smoothing process was necessary to reduce the fluctuation of percentile values. A large sample size of ≥ 300, as we have in our study, was necessary for each age group and sex for precise curve fitting [20]. The model parameters were estimated based on the method of maximum penalized likelihood. We have estimated the 3^rd, 5^th, 15^th, 50^th, 85^th, 90^th, 95^th, and 97^th percentiles for head circumference-for-age, length-for-age, and weightfor- age. Data analysis was performed using the LMS Chart Maker Light, version 2.54 (Medical Research Council, London, UK).

Results

The study sample comprised 542 boys and 546 girls and each participant had 7 measures of anthropometric indices at birth and follow-up time points i.e., 3, 6, 9, 12, 18 and 24 months. About 50.2% of total studied children were girls and 59.4% of them were the first child of their family. 682 (63%) of fathers and 842 (78.4%) of mothers had college education. About 30% of the mothers were employed. Delivery by cesarean section was estimated to be 73.2%. The mean (SD) age of mothers and fathers were 33.17 (4.4) and 37 (5.2) years, respectively. The mean (SD) gestational age of the Isfahan children was 38.2 (1.14) weeks. More details about the characteristics of children and their parents have been presented in Table 1.

Table 1: Characteristics of studied participants

Figures 1-3 present growth reference charts of Isfahan children aged 0-2 years at 3^rd, 5^th, 15^th, 50^th, 85^th, 90^th, 95^th, and 97^th percentiles for head circumference-for-age, length-for-age, and weight-for-age for both girls and boys and the WHO growth standards counterparts. Tables 2-4 present the percentiles for head circumference for age, length for age, and weight for age for both girls and boys from birth to 24 months.

Figure 1. Head circumference percentiles (P3, P15, P50, P85, and P97) by age for girls and boys ages 0-2 years that were born in Isfahan compared to the WHO growth standards

Figure 2. Length percentiles (P3, P15, P50, P85, and P97) by age for girls and boys ages 0-2 years who were born in Isfahan compared to the WHO growth standards

Figure 3. Weight percentiles (P3, P15, P50, P85, and P97) by age for girls and boys ages 0-2 years who were born in Isfahan compared to the WHO growth standards

Table 2: Percentiles of head circumference (cm)-for-age for girls and boys, age in months

Table 3: Percentiles of length (cm)-for-age for girls and boys, age in months

Table 4: Percentiles of weight (kg)-for-age for girls and boys, age in months

Head circumference for age

Median head circumference at birth, 3, 6, 9, 12 and 18 months was 34.40, 39.69, 42.41, 44.27, 45.29 and 46.79 cm, respectively for girls. Median head circumference at birth, 3, 6, 9, 12 and 18 months was 35.05, 40.77, 43.56, 45.39, 46.49 and 48.01 cm, respectively for boys, (Table 2). As can be seen head circumferences of boys were bigger than girls of the same age in all percentiles and difference ranged 0.6-12 cm.

Figure 1 compares the percentiles (P3, P15, P50, P85, and P97) of the Isfahan child growth references for head circumferences by age for girls and boys, and the WHO growth standards. Differences in the percentiles of head circumferences between the Isfahan references and the WHO standards tended to increase with age. Overall, the percentiles of the Isfahan references were higher than WHO standards, and the differences between two growth references ranged from 0.1 to 1.1 cm.

Length for age

Median length at birth, 3, 6, 9, 12, 18 and 24 months was 49.58, 60.63, 66.75, 71.64, 75.44, 82.22 and 87.43 cm, respectively for girls. Median length at birth, 3, 6, 9, 12, 18 and 24 months was 50.23, 62.08, 68.37, 73.16, 76.92, 83.52 and 88.73 cm, respectively for boys. Boys were taller than girls of the same age in all percentile (Table 3).

Figure 2 compares the percentiles of the Isfahan child growth references for length by age for girls and boys, and the WHO growth standards. Similar to head of head circumference, differences in the percentiles of length between the Isfahan references and the WHO standards tended to increase with age. Overall, the percentiles values of the length references of Isfahan children were bigger than the WHO standards, and the differences between two growth references were about 1.5 cm, especially after three months of age. As depicted in Figure 2, the widths between the 3^rd and 97^th percentiles for length rapidly increased with increasing age in both girls and boys in both growth references.

Weight for age

Median weight at birth, 3, 6, 9, 12, 18 and 24 months was 3.10, 5.87, 7.44, 8.46, 9.07, 10.40 and 11.40 cm, respectively for girls while for boys was 3.25, 6.41, 7.99, 9.04, 9.68, 11.01 and 12.08 cm (Table 4). As it was expected, boys were heavier than girls of the same age in over the all percentiles, with differences ranging from 0.1 kg to nearly 0.7 kg.

According to Figure 3, there is no substantial difference between the percentiles of weight of the Isfahan references and the WHO standards for boys, however, the 3^rd percentile of the Isfahan references for girls were above than the WHO standards. Similar to with those we observed for length, the widths between the 3^rd and 97^th percentiles for weight increased with age in both girls and boys in each of the two growth references.

Discussion

We developed growth reference charts for head circumference, length, and weight, for age based on a representative cohort of children aged 0-2 years which born in Isfahan city, central of Iran. According to our findings, the head circumference, length, and weight percentiles values for boys were higher than those of girls in all age groups. Compared with the WHO standards, Isfahan children had similar patterns of weight growth but were taller in all age groups and also had bigger head circumference.

We adopted WHO guideline for constructing the growth reference charts; consequently, the prevalence rate of low birth weight (<2500 g) was 2.9% for Isfahan children that it was in accordance with the WHO standards (2.1%) [6,18].

The patterns of Isfahan's children weight growth showed similarity with those of the WHO standards. One possible explanation is that, in the current study we included only children who were exclusively or predominantly breastfed for at least 4 months similar to the WHO standards. According to results of other studies, breastfeeding has protective effects against obesity in children [21]. Payande et al. study also showed that the weight growth curves of Iranian children were similar to WHO reference curve for Infants between 0-24 months [22].

Different previous studies in different countries found that growth charts differed from the WHO growth standards with regard to height of children. For example, the results from WHO sample children were taller than the CDC and Japanese children population and it is expected to obtain higher prevalence stunting (<-2 SD) of if WHO references are used; however, in population such as China, Poland and Germany the cut-of value for depicting stunting were higher than those reported values by WHO; resulting underestimate of stunting prevalence in these nations when WHO references are applied [23-26]. According to the observed difference between our study results in terms of length for age and the higher centiles of our study than corresponding WHO it is expected to have underestimates of stunting for our local population by applying WHO references. Such differences in various studies have been attributed to variations in inclusion criteria for included children, ethnicity and sample sizes, environmental and nutritional factors have caused the divergence in growth patterns.

According to our study findings, the centiles of the head circumference in male children was larger than the female ones. Also, although based on a visual comparison and tracking the patterns of curves semblance was observed to that of the WHO; however, our sample children in both genders were found to have a bigger head circumference size compared to the WHO charts. Few studies have been conducted about head circumference on Iranian infants and some scare recent studies have attempted to evaluate the differences in the size of head circumference between countries as well as between generations within the same country [27]; as our study showed large difference with WHO references; these differences emphasize the need to perform periodical re-evaluations head circumference size standards in different population. Head circumference growth evaluation should be considered as a dynamic process over a period of time which it will provide useful information about growth of both the bones and the brain as the major stimulus for skull growth [27].

Although Isfahan's references for weight were similar to those of the WHO standards, the head circumference, and length growth curves demonstrated by Isfahan children were different substantially in some centiles and ages from WHO standards. Findings regarding the difference between our references and WHO standards in terms of these indices suggest that the prevalence of growth deficiency in this local population will be lower if assessed by the WHO standards. Therefore, these results reflect imprecision and inapplicability of the international references such as WHO and CDC reference and support the need for local child growth references, in order to better monitoring the growth and development of children in Isfahan and consequently in Iran. However, further research is necessary to investigate the length and head circumference growth trajectories of Iranian children and the determinants of said trajectories.

It is important to recognize some strengths as well as potential limitations of the present study. This survey can be considered as the first study that by using a relatively large sample of healthy children construct growth references for all the three anthropometric measures (head circumference, length, and weight) for children aged 0-2 years longitudinally and prospectively in a largest city of Iran as representative of center of Iran population. We followed the standards and reliable international protocols that they suggest large sample size with 400 to 500 observations in each age-gender group allows the calculation of stable and precise estimates at the percentiles [28]. Most of the previous studies in Iran were based on low sample size or they did not consider all the three anthropometric measures, simultaneously, particularly in a longitudinal approach. We constructed references for children aged 0-24 months based on WHO's inclusion criteria, therefore, the Isfahan references and WHO standards were completely comparable. The LMS methodology used in this study is the most common method by the WHO and the CDC and also other countries to construct children ’ s growth references. Despite these strengths, the anthropometric data in this study collected from the medical record of each child although had been measured by professional health staff in the health centers.

Conclusions

This study developed Isfahan growth references to monitor the growth of children between 0-24 months using longitudinal cohort data. These growth references are the most recent, and comprehensive charts of the Isfahan population of children. The head circumference and length growth patterns of Isfahan children were different from those “ prescribed ” by WHO standards. These findings suggest that caution should be taken when applying WHO standards to Iranian children. The accessibility of such reference charts should help clinicians and healthcare providers in the Isfahan for better monitoring of children. Accordingly, we recommend these growth reference charts to be used by all health centers and hospitals of the Isfahan city. Also we strongly encourage further similar studies in Iranian population to establish national growth reference charts.

Funding

This study was supported partly by Isfahan University of Medical Sciences (Project numbers: 193122).

Acknowledgments

We appreciate Mrs. Zamane Vafaei, Mr. Hosseingholi Najafi, Mrs. Marzieh Heidari, and the Vice Chancellor for Health and Isfahan Provincial Health Center and all staff of Health Centers of Isfahan for their excellent cooperation.

Conflict of Interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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