Hong Kong Journal of Paediatrics [HK J Paediatr (New Series) 1999;4:3-9]

Department of Paediatrics, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Hong Kong

SYM Tam ( 譚月明 ) RN, BhSc
JPE Karlberg ( 賈仕博 ) MD, PhD
EYW Kwan ( 關彥華 ) MBBS, MRCP, FHKAM, FHKCPaed
AMC Tsang ( 曾雯清 ) MBBS, MRCP, DCH, FHKAM
FM Baber MBChB, FRCP(Edin)
LCK Low ( 盧忠啟 ) MBChB, FRCP, FHKAM, FHKCPaed, FRCPCH

Correspondence to: Dr LCK Low

Received Aug 31, 1998

The growth, health and socioeconomic status of two cohorts of Hong Kong Chinese infants in the first two years of life were compared. Cohort I included 578 infants born in 1967 and cohort II comprised of 48 infants born in 1994. The growth data of these two longitudinal studies were compared with the National Center for Health Statistics (NCHS) growth reference. An improvement in growth in terms of weight, length and head circumference had been demonstrated in children between these two periods. When compared to the NCHS reference means, growth faltering in mean weight, length and head circumference was observed in cohort I between 6 and 18 months. The changes in mean weight, length and head circumference in Cohort II were close to the NCHS reference mean values. The children at 2 years of age in the 1994 study were 5.4 cm taller than children in the 1967 study. The improved health and socioeconomic status in children in the present study have been paralleled by the improvement in growth over the past 27 years. We conclude that growth faltering in early life is influenced by socioeconomic factors rather than by differences in ethnic background.

Introduction

Children are getting taller and bigger over generations because the pattern of growth and somatic development of children changes over time.^1-2 The size of children tends to improve with favorable exogenous factors like better nutrition, health and socioeconomic conditions.^3-5 Secular changes in growth and tempo of growth are indicators of the socioeconomic and health status in the population.^1,6

The correlation between the secular changes in growth and the socioeconomic status in many industrialized and developing countries have been studied. In the Netherlands, the positive secular growth change in Dutch children began at the start of gradual economic development in the 1850's.¹ In Japan, the improvement of physical status in children started in the years right after the end of World War II.^7-9 In Norway, the secular growth trend of school children was interrupted by the deprivation during World War II under Nazi occupation.¹⁰ In China, there has been a significant increase in body height and weight in Chinese children. This took place after the prolonged civil war between 1930's to 1949 and the founding of the People's Republic of China in 1949 leading to the recovery of the national economy.¹¹

With the rapid socioeconomic improvement in Hong Kong in the last thirty years, a positive secular change in growth in Hong Kong Chinese children has been observed based on several cross-sectional growth studies in children and adolescents (3-18 years of age) born in the 1960's and 1990's.^12-16 The environmental and socioeconomic factors leading to the secular change in growth have not yet been described and a relevant publication is still under preparation by Dr F Baber.¹⁷ There is a paucity of studies investigating the secular change in growth in Chinese children in the early year of life that has been highlighted by Tanner to be a critical period of physical growth.⁶ In addition, Liestøl⁵ reported that the tempo of growth depends markedly on socioeconomic conditions in early life, while the conditions at higher ages are of less importance. The aim of the present study is to describe the secular change in growth, and the changes in socioeconomic and health status in two cohorts of Hong Kong Chinese children in the first two years of life born in 1967 and 1994.

Materials and Methods

This was the first longitudinal growth study of Hong Kong Chinese infants in Hong Kong conducted by Field and Baber in 1967.¹⁸ Recruitment of the study babies was done in the two regional hospitals Kwong Wah Hospital and Queen Elizabeth Hospital. Chinese newborn babies with body weight more than 2.27 kg were randomly recruited. Babies with known abnormalities were not included in the study. 36.7% of the eligible babies could not be included because the parents refused to allow the babies to participate in the study.

The study included 782 normal newborn babies at the start of the survey. 578 babies remained in the study at their 2 years of age. Among these 578 babies, 55% were boys and 45% were girls. Around 6% of the babies were low birth weight (<2.5 kg) babies and the mean birth weight was 3.08 ± 0.39 kg and 3.03 ± 0.35 kg for boys and girls respectively. 95.4% of the baby's parents originated from southern China while others originated from other parts of China. The babies were followed up every month in the first year and then every 2 months in the second year. This was an extensive study on the pattern of growth, nutritional status, development and mode of rearing children in Hong Kong. In addition, a detailed description in the socioeconomic status of the families was included.

Healthy newborn babies were randomly recruited in the postnatal ward in Tsan Yuk Hospital and Queen Mary Hospital in 1994. All the babies had a birth weight between 2.7 and 4.0 kg and a gestational age between 37 and 42 weeks. Their parents should be Chinese in origin and permanent residents of Hong Kong. Mothers of the selected babies should be free of pregnancy complications like gestational diabetes mellitus, preeclampsia. Among the eligible parents and babies, 20% of them refused to participate into the study during the recruitment. 61 babies were recruited in the study; most of them were recruited in Tsan Yuk Hospital and 6.4% of them were recruited in Queen Mary Hospital. 98.2% of the baby's parents originated from southern China while others originated from other parts of China.

Forty-eight infants remained in the study at the end of the second year. 52% of these infants were boys and 48% of them were girls. The mean and SD in birth weight in the boys and girls were 3.26 ± 0.28 kg and 3.23 ± 0.24 kg, respectively. Their growth, health and detailed nutritional data were collected every 2 months in the first year of life and then every 3 months in the second year by the same observer in a standardized way. Their body weight was measured to the nearest 1 gm by using an electronic balance. Supine length was measured using a horizontal stadiometer and the mean of five consecutive measurements to the nearest 0.1 cm were taken for each infant. In between each measurement, readjustment to a standard position for measurement as described by Tanner¹⁹ was made whenever necessary. Ninety-eight percent of these measurements were with the standard deviation less than 0.26 cm. The maximum occipital-frontal head circumference was measured by using a non-stretchable tape. Health status was documented after a medical examination at every visit. Medical consultations were provided when the infants were sick. The socioeconomic data on the infants' families were obtained during recruitment. The study included a 2 day prospective food record before each visit and blood sampling for growth factors analysis at 6, 10, 12 and 18 months of age. However the results of the effect of nutrition and growth factors on growth of these infants will not be presented in this paper. The study was approved by the Ethics Committee of the Faculty of Medicine, The University of Hong Kong. Written informed consent was obtained from the parents at recruitment.

All the growth data of the infants in cohort I was obtained from the original raw data set with the permission of the principal investigator Dr Flora Baber. The mean weight, length and head circumference of boys and girls in the first two years of the two cohorts were analysed. These mean values were compared with growth reference of The National Center for Health Statistics (NCHS).²⁰ The differences in mean weight, length and head circumference in boys and girls of the two cohorts from the reference mean were calculated. The individual difference in weight, length and head circumference of cohort II from the mean values of cohort I were test statistically with an expected mean values of zero using Student t-test and signed rank test.

Health data of cohort I was obtained from both the raw data set and the book published by Field and Baber¹⁸ while the socioeconomic data were obtained from the publication only. Health status of the infants and their socioeconomic conditions in the two cohorts were analysed and compared. Additional socioeconomic data were obtained from the publications of the Census and Statistic Department of the Hong Kong Government.^21-29

Result

The socioeconomic status of the study babies' families of the two cohorts was similar to that of the general population in the respective era. In the 1967 study, illiterate parents in the cohort were 24.9% while illiterate people among the Hong Kong population (age over 15) were 25.2%. 25% of the study parents completed their lower or higher secondary education while 24.9% of people age above 15 in 1967 in Hong Kong completed the same education level.²¹ In comparing the economic background, the median income of the study family was $441 per month while the overall average salary for industrial workers in 1967 in Hong Kong was $390 per month.²¹ In the 1994 cohort, 76% and 11.5% of the baby's parents completed secondary and tertiary education, respectively. In 1994, Hong Kong people aged above 15 who completed the respective education level were 50.8% and 17%.²² in comparing the median income of the study parents and the nominal wage index in 1994 in Hong Kong,²³ there was no significantly difference in family income (p>0.05) between the cohort and general population.

The growth of Hong Kong Chinese infants born in 1967 and 1994 in the early life

When the growth of the two cohorts of Hong Kong Chinese infants was compared to the NCHS growth reference in the first two years of life, growth faltering was observed in cohort I between 4 and 18 months while no faltering was observed in cohort II. Fig. 1 shows the change in mean body weight in the first two years of the two cohorts as compared to the NCHS reference. The mean weight values of both boys and girls in cohort II were above the NCHS mean values during the initial six to eight months after birth and then followed by lower values up to 24 months of life. The change in mean weight of the cohort I slowed down from 4 months onwards and deviated towards the 5th centile of the NCHS reference. Fig. 2 depicts the change in mean length of the two cohorts in the first two years as compared to the NCHS reference. The change in length in both boys and girls in cohort II followed the NCHS reference curves closely while that of cohort I deviated from the reference mean towards the 5th centile from 4 months onwards. Similarly, the mean change in head circumference with age in cohort II followed the NCHS mean values closely while that of cohort I slowed down from 2 months and then continued along in the 5th centile of the NCHS reference (Fig. 3a and 3b). The differences in the mean weight, length and head circumference of the two cohorts from the reference means are shown in Fig. 4-6. As compared to children born in the 1960's, the children in cohort II were 5.4 cm taller, 1.2 kg heavier and had a 1.5 cm increase in mean head circumference at the age of 2 years. In comparison of cohort I, the mean weight, length and head circumference of cohort II at every examination age were significantly higher (p<0.05) throughout the first two years of life in boys and girls (Student t-test and signed rank test).

The Socioeconomic status of Hong Kong and the health of Chinese infants in 1967 and 1994

In the 1967 study, about 97% of the fathers and 26% of the mothers were working. 21.3% of the fathers were managers or professional. Around 56.7% and 22% of the father were skilled and unskilled workers, respectively. The majority of working mothers were skilled workers. Some mothers who were housewives had occasional casual work at home to earn money such as embroidery or making plastic flowers. The median family income (income of father and mother) was HK$441 per month.

Twelve families had income below HK$358 per month and 12 of them their income was higher than HK$554 per month. According to the annual report of Hong Kong in 1967,²⁴ the range of mean daily wages at the end of 1967 was: HK$10 to HK$30.1 for skilled workers; HK$6 to HK$22.1 for semi-skilled; and HK$5.2 to HK$14.6 for unskilled. In the 1994 study, only one (2%) father was unemployed. 41.8% of the fathers were skilled workers and some worked as managers or administrators (22.9%) or professionals (14.6%), Thirty-four (71%) mothers had steady employment, while the rest were housewives without any casual work at home. The majority (52.1%) of the working mothers held clerical jobs. The median family income was HK$18,000 per month. Twelve families had family income below HK$12,500 per month and 12 families had family income higher than HK$23,000 per month. The nominal wage index for monthly family income was HK$16,768 in 1994.²³

In the 1967 growth study, housing for the study families was mainly in the government estates, huts on the hillside, rooftop dwellings and private tenement flats divided into several tiny bed-spaces or cubicles. 59.7% of the families lived in only one room and 66% of them had more than three persons sleeping in the single room (up to 11 square meters). Only 18.1% of the families lived in self-contained flats and 3.9% of them lived in squatter huts. 24.8% of the families had three persons living in the household. 68.2% and 20.5% of the families had less than six persons and more than seven persons living in the household respectively. In the 1994 study, all families were living in self-contained flats in residential buildings. They lived in flats with a mean area of 55.4 square meters and mean area of 12.3 square meters per person. Most families were small with an average of 4.9 persons in each household (1/4 of the families had one living-in maid). Twenty-three (31.5%) families had three persons in the household. 74.0% and 10.9% of the families had less than six persons and more than seven persons living in the household respectively.

In the 1967 study, 12.1% of mothers and 2% of fathers were illiterate and had not received any formal education. 24% and 25% of the parents had completed their primary and lower or higher secondary education, respectively. Only 2.2% of the parents had graduated from tertiary institutions. In the 1994 survey, the majority of the mothers (83.3%) and fathers (68.8%) graduated from secondary education. 4.2% of the mothers and 19% fathers completed tertiary education.

In the 1967 study, mothers were the principal care providers of the babies. Eleven percent of the babies were cared for by their grandparents. Some of the babies were looked after by their elder sisters who might only be eleven or twelve years old. 56.2% of the babies were either the first or the second children in the families. The birth order of 31.4% of the babies was more than three. In the 1994 study, most of the infants were the first child in the families. One third (31%) of the mothers took care of their babies while 21 babies (43%) and nine babies (20%) were looked after by their grandparents and maids respectively. 91.6% of the babies were either the first or the second children in the families. There was only one who was the fourth child in the family.

In the 1967 study, the common diseases in infants were respiratory tract infections and diarrhoea. These children were affected by a mean of 2.9 episodes of respiratory tract infections and 0.9 episodes of diarrhoea in the first year of life. In comparison, the infants of the 1994 study had less frequent exposure to these two infections; the mean episode per infant of respiratory tract infection and diarrhoea was 2.1 and 0.6 respectively during the first year. Other infectious diseases like measles were common in the 1960's; 21.2% and 56.5% of infants had experienced measles during their first and second year of life respectively. In the 1994 study, none of the infants had measles in the first two years of life. The incidence of infections of infants in the present study had decreased by almost half as compared to that observed in children in the 1967 study. In the first year, there were 5.4 episodes of infection per infant in 1967 dropping to 2.9 episodes of infection per infant in 1994. In the second year, there were 4.1 episodes of infection per infant in 1967 while those in the 1994 study were 2.9 episodes of infection per infant.

Discussions

This paper presents the growth in the early life of two cohorts of Hong Kong infants followed up by the Department of Paediatrics, the University of Hong Kong in which the cohort recruited in 1967 with a particular reference to growth faltering between 6 and 18 months of age. The socioeconomic status of the baby's families in the 1967 and 1994 cohort showed that the cohorts were representative to the general population in Hong Kong in 1967 and 1994, respectively. Although the numbers of babies recruited in the 1994 study was relatively small, the statistically significance of improvement in growth in the 1994 cohort indicates that the sample size is enough for the comparison. The relatively small sample size in the 1994 cohort could not be regarded as a population-based study, but we think this cohort was reasonably representative of the present day healthy newborn infants. In spite of the inclusion criteria in birth weight of study babies in the two cohorts were different, only 6% of the babies in the 1967 cohort were low birth weight babies and 11.4% of the babies had a birth weight between 2.5 and 2.7 kg (the lower limit in birth weight in the 1994 cohort). Exclusion of such infants from our analysis would not alter the conclusion we have reached.

There was a dramatic improvement in the economic status, accommodation, education of parents, care and health status of children in the 1994 study as compared to those observed in the 1967 cohort. The growth in economy in Hong Kong was the crucial factor. The economy maintained a steady growth from 1967 to 1994 with the per capita gross domestic product growing by ten times; it increased from HK$12,000 in 1967²⁵ to HK$120,000 in 1994²⁶ (in constant market prices). The families in the present study were better off financially than those in the previous study. The median family income in the 1994 study was 40 times more than that of the 1967 study while the annual average composite consumer price index had risen only by 36% between 1967 and 1994.^26-27

Overcrowding has always been a big problem of living in Hong Kong. However, the living conditions of children in the 1994 study had markedly improved as compared to that in the 1967 study. At least, all the survey families lived in self-contained flats in residential buildings and each person in the family had much more space in the home. The educational level was low in the parents especially the mothers in the 1967 study. The educational level of the populations changed after the nine-year compulsory education schemes were implemented in 1977. The number of people entering tertiary education increased to 17.1% of the population in 1994.²² This improvement was reflected in the educational standards of parents in the 1994 study.

In the 1967 study, almost half of the babies were the third or in one instance the tenth child in the family. It was because family planning was not widely practiced in the community in 1967 with a crude birth rate of 37.1 per 1000 population.²¹ With such large families in the 1967 study, the mothers had to perform a lot of house work in addition to taking care of several babies and children at the same time. For working mothers, the grandparents or even the older siblings would have to take care of the younger siblings and help with the housework. The quality of child care would frequently be unsatisfactory in such situations. In recent years, most couples had just one to two children. The crude birth rate had dropped to 12 per 1000 population in 1994.²² Although there were still a lot of working mothers in the present study, grandparents or maids could concentrate their tender loving care to just one or two children in the families.

The health status of children in the present study improved tremendously. It has been reflected in the dramatic decrease in infant mortality rate. The infant mortality rate had dropped from 26 per 1000 livebirths in 1967²¹ to 4.8 per 1000 livebirths in 1994.²⁶ Fig. 7 showed the decrease of infant mortality rate from 1966 to 1996 ^21,26,28-29 The infection rate of children in the 1994 study dropped to almost half compared to the children in the 1967 study. The comprehensive vaccination programmes and the improved health care provided by the Hong Kong Government through Maternity & Child Health Clinics, Outpatient Clinics and public hospitals contributed to this decline. There has been a significant increase in the number of registered doctors. In 1967, Hong Kong had 0.43 registered doctor per 1000 population²¹ and this had increased to 1.1 registered doctor by 1994 (the population increased by 50% in the same period).²⁶

In Hong Kong, three detailed longitudinal growth studies in early childhood have been done from 1967 to 1994. In addition to the two studies mentioned in this manuscript, another longitudinal population study was conducted by Leung et al in 1984.³⁰ When comparing the studies in 1967 and 1984, a positive secular change in growth had been demonstrated in children between these two periods. The children at 2 years of age in 1984 had a mean length which was 3.8 cm longer than that observed in children in 1967. An improvement in growth continued between 1984 and 1994; the children in the present study were 1.5 cm longer than children in the 1984 study. The effect of the secular changes in growth on the final adult height in any population can only be realized after more than one generation.

Davies and Leung (1985)³¹ described a distinct growth faltering in Hong Kong Chinese children in 1963 and 1978 compared with Caucasian children using the NCHS reference. As growth faltering is also commonly found among the "elite" subset of other Asian populations, they speculated that growth faltering observed in Asian children could be due to ethnic differences. The healthy full-term infants of the 1994 cohort, who could be hardly regard as a "elite" group, had no growth faltering. The mean weight, length and head circumference of the present day Hong Kong Chinese children in the first two years of life were similar to those of the Caucasian children. Their mean weight was even higher than that of the Caucasian children in the first six months of life. Thus, the growth faltering previously observed in Hong Kong Chinese children relative to the NCHS reference was unlikely to be due to ethnic difference. The rapid socioeconomic improvement in Hong Kong followed by the better health status have been paralleled by the improvement in early growth in our children over the past 27 years. The socioeconomic factors and infections in early life as well influencing growth. The result of nutritional analysis showed an improvement in the nutritional status in children of the 1994 study compared to thirty years ago (manuscript in preparation). A study to correlate nutrient intake and growth in Hong Kong Chinese children in these two periods is in progress.

Acknowledgment

This study was supported by the Faculty of Medicine, The University of Hong Kong (CRC grant # 337/045/0009 and # 335/045/0019), the Health Services Research Grant- long term health consequences of insults in early life and the Edward Sai-Kim Hotung Paediatric Education and Research Fund, Department of Paediatrics, The University of Hong Kong.

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