An analysis of birth weight by gestational age of infants born in England and Wales, 1967 to 1971.

Abstract

SummaryFrom a sample of 271,519 births occurring in England and Wales from 1967 to 1971 tables have been prepared describing the relationship of birth weight to gestational age, subdivided by the sex of the infant and parity of the mother. The sample was composed of single babies born live in National Health Service (NHS) hospitals at gestational ages of 28 to 44 weeks and they comprised approximately 8.5 per cent of all such births occurring in NHS hospitals during the study period. No secular trend in birth weight was observed. The distribution of birth weights at a given gestational age was normal above 36 weeks but skewed or bi‐modal in preterm infants. The non‐normal distribution of preterm infants could be described accurately in terms of a mixture of two normal distributions with different mean values but the same standard deviation. The population with the higher mean birth weight comprised approximately one‐third of the observations each week between 28 and 34 weeks but was only 0.79 per cent of all births. The mean birth weight varied between 3.10 and 3.37 kg. equivalent to that of term infants of 38 to 39 weeks' gestation. The sex ratio was similar to that of term infants. The sex ratio (the number of single male infants born alive for every 100 female infants) of the population with the lower mean birth weight rose with increasing prematurity and was 127.9 between 28 and 34 weeks. It is concluded that the population of infants with the higher mean birth weight is composed mainly of pregnancies in which the length of gestation was mistaken. The error in gestational age is not a multiple of months but is continuously distributed. These pregnancies form a small fraction of all births but an important fraction of preterm births.Tables of mean birth weight (±1SD), prepared after correction for the nonnormal distribution of preterm birth weights, gave results which were similar to those reported earlier for infants born in Aberdeen. Male infants were heavier than females and infants of multiparae heavier than infants of primiparae at most gestational ages. Centile tables have been prepared for male and female infants which may be useful to those who wish to monitor the birth weights of infants born in England and Wales.