Growth patterns from birth to 24 months in Chinese children: a birth cohorts study across China

The assessment of child growth is important in detecting under- and over-growth, which can provide information for timely intervention. The first 1000 days of life (from conception to 2 years of age) is a period of rapid growth and development, and vulnerable to nutritional and environmental influences [1]. Identifying normal child growth patterns is of fundamental importance in growth assessment.

Both the World Health Organization (WHO) growth standards [2] and the China growth references [3] are being applied in China. The WHO growth standards for children aged 0 to 24 months were constructed based on longitudinal data of children (n = 882) by using selection criteria of having socioeconomic conditions favorable to growth and having access to breastfeeding support (for qualifying as “standard”) from the WHO Multicenter Growth Reference Study (MGRS) conducted in six countries from 1997 to 2003 (without a site in China). The China growth charts were constructed from a large (n = 44,250) cross-sectional study based on stratified random sampling of children in nine cities of China, which was conducted from May to October in 2005 [3]. Comparison of the growth curves over the restricted range of ages from 0 to 2 years indicated the reference for China was significant higher for BMI for boys and girls. However, the comparisons were complicated by differences in inclusion/exclusion criteria (for the WHO sample, strict criteria about known constraints on growth and cooperation with feeding recommendations, which led to over 80% of mother-infant pairs being ineligible; for the China sample, multistage stratified cluster sampling was used based on urban/suburban areas, districts, and community, with several exclusion criteria), as well as by differences in the design of the studies (longitudinal for the WHO study and cross-sectional for the study in China). The difference between China growth references and WHO growth standards could have been an artifact, so confirmation study is warranted.

Historically, in some circumstances, secular trends of height have occurred from one generation to the next generation [4]. China has a diverse population, environment, dietary habits and tradition, and it is going through rapid modernization and urbanization. Early child growth has drawn much attention since these factors may affect growth. China started the 1st National Survey on the Physical Growth and Development of Children (NSPGDC) in the nine cities of China in 1975, and conducted the survey every 10 years from 1975 to 2005 to address possible secular trends, with the most recent data (from 2005) providing the current references for growth in China [3] (but in need of a 10-year update in 2015). Longitudinal data from a sample with stricter inclusion/exclusion criteria would provide a better comparison to the WHO standards. A small cohort [5] recruited in 2007 (n = 1531 retained up to 1 year of age) with strict WHO criteria applied showed significant differences (heavier in weight, longer in length, and bigger in head circumference) compared to WHO standards, as well as compared to the current cross-sectional references, which showed similar differences (except for the 97th percentiles that were lower rather than higher).

Long-term follow-up data has enormous value in evaluating the optimal individual growth trajectory, which may not be captured by cross-sectional data [3, 6]. Between 2012 and 2014, six longitudinal birth cohort studies were launched in China. A number of common exposures shared by all cohorts were collected and common outcomes were observed, which formed the foundation of China Birth Cohort Consortium (CBCC). This collaboration provided, for the first time in China, longitudinal growth data from birth cohorts from various regions of the country, but it still is a convenience sample from an efficient combination of cohorts.

This report examines growth patterns from birth to 24 months in Chinese children by pooling the individual level anthropometric follow-up measures from CBCC. The growth references from the 2015 CBCC will be used for comparison to the 2006 WHO longitudinal growth standards and the 2005 China cross-sectional growth references to provide an update on how healthy infants are growing in modern China.

This report presented the z-score curves of 4251 children who were born at term to mothers without gestational or preexisting diabetes, chronic hypertension, preeclampsia, or eclampsia. A total of 28,298 anthropometric measures were obtained from ages 0 to 27 months (Additional file 1: Tables S2 and S3). All were urban children. 51.1% were boys and 54.0% were delivered via C-section. The mean maternal and paternal height was 161.4 (SD 4.9) cm and 174.4 (SD 5.3) cm, respectively. Mean (pre-pregnancy) BMI was 20.6 (SD 2.8) kg/m² for mothers and 23.9 (SD 3.3) kg/m² for fathers. As expected, boy infants had greater birthweight, length and head circumference than girl infants (Table 1). Most (80.3%) of mother had college education or more and 98.3% of mother were Han ethnicity. During the first 6 months, most (77.6%) of infants were mixed fed, and 13.4% had exclusive breast-feeding. In the first 2 years, 27.9% of children were exposed to passive-smoking. There was no sex difference for these factors (Table 1). Over the follow-up assessments (see Fig. 1), the children aged 0 to 2 years in this cohort were taller, heavier, and had greater head circumference than the children in the WHO cohort.

The height- and weight-for-age values were higher in our longitudinal cohort assessed in five cities of China (Shanghai, Ma’anshan Anhui, Wuhan, Jiangsu, and Guangzhou) than in the cohort based on a cross-sectional study in nine cities of China (Beijing, Shanghai, Harbin, Xi’an, Nanjing, Wuhan, Guangzhou, Fuzhou, and Kunming) [3]. This could be a secular trend. The CBCC cohorts recruited pregnant women in provincial or large tertiary maternity and child hospitals. Most mothers had high education (college or higher), maternal smoking was rare, and the living standard were relatively high. Thus, the growth data in this study may reflect infant growth patterns under near-optimal circumstances. Since our data were acquired recently (10 years since 2005), the higher length and weight may also reflect an ongoing secular trend [4]. The WHO data suggest that secular trend may depend on where the cohort was acquired: the predicted adult height from the child’s length at 2 years suggested there would be no parent-offspring difference in Norway and the United States (i.e., no increase due to a secular trend), but the predicted adult height was much larger than mid-parental height for the other four countries (Brazil, Ghana, India and Oman). [15] Based on the taller height reported here for ages 0 to 24 months than the 2005 China data, we expect a secular trend (i.e., we predict that average adulthood the height of the children in China will exceed the average height of their parents). While China has undergone dramatic progress in economic and social development, the differences still exist between urban and rural areas, different ethnics, and different social economic. The growth pattern observed in this study may reflect infant growth patterns under more optimal circumstances.

Some studies have found that some child population might have their own growth pattern [16], and our study confirmed that Chinese children may be one of them [3, 17, 18]. The difference in values for height-, weight- and BMI-for-age, weight-for-length, and head circumference in this report in comparison to the WHO standards suggests an interesting country difference, and adds to previous comparison that have been summarized in a recent review [19]. Based on studies from both longitudinal and cross-sectional designs, this review concluded that the WHO standards for height and weight “… endorsed slenderness in the midst of an obesity epidemic” and for head circumference were underestimates (and “… would put many children at risk for misdiagnosis of macrocephaly and microcephaly”). Healthy children in some countries are classified (perhaps inappropriately) as “stunted” [16]. In opposite of findings from some countries (overestimating stunting) [16], overall, our study confirmed that the values of growth measures were higher for the key z-score cutoffs in Chinese children in comparison with WHO growth standards [3, 5].

Our references provide the potential cutoffs for evaluating child growth in a population (like in modern China), where children are the center of attention in the family and are growing under favorable environments. Length has been widely used in early detection of stunting, while weight is commonly used as a measure responsive to short-term influences [20]. Head circumference is then the next most-used measure in clinical settings. To reflect the growth centile (position) of a Chinese child in local population, conditioned on age and sex, the Chinese growth standards need to be considered. It may help identify the infants who suffer from poor and modifiable conditions, and thus target those who may benefit most from intervention. In this study, while another term was considered (“growth pattern”), the term “growth reference” was used to maintain consistency with the term used in other publications about Chinese cohorts and to contrast to the term “growth standard” used for the WHO cohort.

One characteristic of this study (the large-scale multicenter prospective birth cohort design) allows us to obtain data on pre- and perinatal risk factors including GDM, chronic hypertension, pre-eclampsia and preterm status. Based on this strength, we could exclude affected mother-infant pairs cases at risk for abnormal patterns of child growth. In this study, the difference of mean paternal age among the three groups of children (mothers with GDM, born preterm, and healthy children) is interesting. Older fathers have more de novo mutations in DNA, and this probably contributes to growth in some cases [21]. Another strength of this study is the longitudinal rather than cross-sectional design. Additional longitudinal analysis [22, 23] of these longitudinal data could better capture and describe the tempo of growth, but due to space limitations will be presented elsewhere. Also, in this sample the educational level of mothers was high, and few of the mothers smoked, so the children lived in advantaged condition, and approach the criteria used for establishing the WHO standards (reflecting how children should grow). Therefore, the data here may reflect growth in near-optimal conditions in China, and provide a growth pattern for contemporary Chinese children.

On the other hand, one limitation of this study is that in some cases head circumference at birth was not measured, and some of children were just followed up to 12 months, which reduced the sample size for this measurement. However, our sample size is still larger than the sample sizes in similar longitudinal birth cohort studies conducted in other countries. We have also performed sensitivity analysis to summary the 3rd, 10th, 50th, 90th and 97th percentiles of all growth measures in infant who had all observations up to 24 months (i.e., without missing observations) and the results were similar to those from all observations (data not shown). Thus, the missing data should be “at random” [9] Also, the birth measures obtained from medical records may not be ideal despite of the high number of the participating hospitals (which were all provincial or large tertiary maternity and child hospitals). Thirdly, this was a convenience sample without specific entry criteria as in the WHO study.

The growth curves in this study represent the growth pattern of today’s normal Chinese children, and may provide references for evaluation of the individual growth status of children growing up in modern China.