Updated gestational age specific birth weight, crown-heel length, and head circumference of Chinese newborns. (Original Article).

Hong Kong Neonatal Measurements Working Group

Objective: To construct gestation specific standards of birth weight, crown-heel length, and head circumference of Chinese infants.

Design: A prospective cross sectional population study.

Methods: The birth weight, crown-heel length, and head circumference were prospectively measured using standard equipment in newborns delivered at 24-42 weeks gestation in the maternity units of 10 public hospitals and two private hospitals in Hong Kong. The findings were used to construct gestation specific standards of these variables. The LMS method using maximum penalised likelihood was used to perform model fitting. The results were compared with those obtained from a cohort of infants born in the same locality between 1982 and 1986.

Results: From October 1998 to September 2000, a total of 10 032 infants were measured, representing 9.6% of the total deliveries in Hong Kong during that period. An extra 307 infants with gestation [less than or equal to]35 weeks were recruited from October 2000 to June 2001. Each of the three variables showed a normal distribution at each gestational week. Gestation specific reference standards for each variable were constructed for male and female infants separately. Comparison with the 1982-1986 cohort showed a significant secular trend to increased birth weight. The trend was small, but significant, for crown-heel length and head circumference.

Conclusion: These growth standards will provide useful references for the care of newborns of ethnic Chinese origin. These standards, especially that for birth weight, should be updated regularly.

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Birth weight, head circumference, and body length of newborn infants are important clinical indicators widely used for evaluation of prenatal growth and identification of infants that require detailed assessment and close monitoring during the neonatal period. Infants whose birth weights are too low or too high have higher mortality and morbidity than those of appropriate weight for gestation and increased risk of complications such as peripartum asphyxia, birth trauma, congenital malformations, and hypoglycaemia. (1-5) Body length is also of prognostic significance: an infant who is underweight but of normal body length has normal growth potential, but a small infant with small body length probably has impaired growth potential because of genetic factors or infectious or other teratogenic insults in early fetal life. (6 7) A recent study has shown that body length is also a predictor of perinatal mortality, with long infants being at higher risk of perinatal death. (8 9) Infants born with excessively small o r large heads may have malformation of the central nervous system secondary to genetic or chromosomal abnormalities or teratogenic insults that carry grave prognostic implications.

Hong Kong, a British colony until July 1997, is a special administrative region of The People's Republic of China. Situated on the southern coast of China, 95% of its 6.8 million population are ethnic Chinese, most of whom are descendants of migrants from the southern provinces of China. (10) The reference standards for birth weight, crown-heel length, and head circumference used in the region were those provided by Lubchenco et al (11 12) 2 until 1987, when local reference charts became available. These local references were based on measurements obtained from 8445 ethnic Chinese newborns born in Hong Kong between November 1982 and January 1986. (13) Over the past decade, the region has evolved from an industrial city to a commercial and financial centre. Associated with this change has been a rapid improvement in the standard of living and health indices, including infant and neonatal mortality. At the same time, the population has experienced an increasing growth rate from 1.1% in 1991 to 2.41% in 1996, an d a falling birth rate from 11.7 per thousand in 1991 to 7.4 per thousand in 1999. Most of the population increase has been due to net inflow of people, mainly from Southern China, which accounted for 24.7-87.9% of the net population increase in the 1990s. (14) Over this time, health workers caring for newborns have noticed a trend of increasing size at birth of the local infants. A pilot study that we performed on 1350 full term newborns delivered in the Prince of Wales Hospital from January 1996 to April 1997 showed that the infants had greater gestation specific mean birth weight, crown-heel length, and head circumference than 15 years previously. In the light of these observations, a working group was formed to carry out a prospective study in 12 maternity units in the territory to establish a set of updated references for local infants.

SUBJECTS AND METHODS

The study was approved by the ethics committee on clinical research of The Chinese University of Hong Kong, and the ethics committees of the participating hospitals. In Hong Kong, all births take place in the maternity units of 20 hospitals. During the study period, about 70% of the newborns were delivered in the 10 public hospitals; the remaining 30% were born in the 10 private hospitals. To ensure that the sample selected truly represented the newborn population in Hong Kong, the babies were recruited from the maternity units of all 10 public hospitals and two randomly selected private hospitals.

All measurements were carried out by two teams of field workers, each consisting of two investigators who had received training in the use of all measuring equipment. The precision of their measurements was assessed by establishing the interobserver agreement of the measurements obtained from the first 100 infants. In random sequence, the teams were stationed in each of the participating public hospitals for two months, and attempts were made to capture all eligible infants born during that period. Thus the study would capture about one sixth of the annual deliveries in each of the hospitals. The antenatal history and condition of each infant were carefully evaluated. A data sheet was used to document parental data, as well as the medical and pregnancy history of the mothers. To obtain a reasonable sample of infants born in the private hospitals, measurements in the two participating private hospitals took place over one year. Logistically it was not possible to include more private hospitals, in which the ne wborn infants were under the care of a large number of private obstetricians and paediatricians.

The main study lasted for two years from October 1998 to September 2000. At the end of two years, it was realised that the number of infants < 35 weeks gestation was relatively small. The study was then extended for nine months until June 2001 to enroll more preterm infants.

Inclusion and exclusion criteria

Singleton newborns of ethnic Chinese origin of gestation 24-43 weeks were eligible for the study provided that informed consent was given by the parents. Infants with the following conditions were excluded: moribund condition at …