|Causes and Mechanisms of Linear Growth Retardation (International Dietary Energy Consultative Group - IDECG, 1993, 216 pages)|
|Psychosocial adversity and growth during infancy|
The investigation was a prospective longitudinal survey of virtually all infants born during one calendar year within an inner city health district in London, England. The district has an ethnically diverse population (140,000), which in socioeconomic terms is relatively homogeneous and quite severely disadvantaged (South East Thames Regional Health Authority, 1984).
The sampling frame comprised all 2510 births that were registered with participating child health clinics or family doctor practices in 1986. The planning and execution of the survey was facilitated by the good relationship built up between the research team and the local community paediatric services in the course of previous research there (e.g. Dowdney et al., 1987).
The selection of subjects who had growth faltering was made from all children who continued to live in the district until 12 months of age, and who were weighed on at least one occasion. During this period 14.4% of the population moved out of the area. There were also missing data on a small minority of subjects (1.2%) who were known to health visitors but were seen only at home. Other families were untraceable (3.3%). A small proportion of infants (0.8%) are known to have died.
Growth trajectories were computed from weight data recorded at clinic visits and expressed as Z scores (Hamill et al., 1979; Jordan, 1986). Scores have been interpolated to target ages 4 weeks, 6 weeks, 3 months, 6 months, 9 months, 12 months and 15 months. In virtually all cases the interpolations were made where at least one of the data points was within 1 month of the target age. Quadratic (three point) interpolation was undertaken by means of the NEWLONG program developed by Carter (1985).
2.1. Definition of growth faltering
Potential cases were limited to singleton deliveries at term, i.e. between 38 and 41 completed weeks gestation. Preterm infants (gestation 37 weeks 6 days or less) were excluded because of the known association with below average growth in the early postnatal period (Brandt, 1986; Ounsted, Moar & Scott, 1982). Also excluded were infants with severe intra-uterine growth retardation (i.e. birthweights at or below the 3rd centile, on charts standardised for gestation, sex, ordinal position, maternal height and mid-pregnancy weight; Tanner & Thomson, 1970). Confirmed cases of growth faltering had to have a weight for age Z score (WAZ) below -1.88 (corresponding approximately to the 3rd centile). This trajectory relative to population norms had to be attained by 12 months of age, and sustained for three months or more. All children with a suspicious weight gain trajectory, on the basis of clinic data, were traced and visited at home for confirmatory anthropometry.
There were 1554 potential subjects remaining after exclusion criteria had been implemented. Of those, 52 (3.3%) were confirmed cases of growth faltering at 12 months of age. Three of these were excluded because of an overt organic disorder that was considered to be making a major contribution to the infant's poor rate of growth. Further information about the study design and outcome can be found in Skuse, Wolke & Reilly (1992).
2.2. Pattern of growth in first year
Preliminary exploration of the data led to the conclusion that the children could be approximately divided into two subgroups, those in whom the onset of growth failure was before 6 months of age, and those in whom it was later.
This was done according to whether the difference in weight between birth and 6 months was greater than or less than half the difference between birth and one year. The former suggests that the greater part of the faltering had occurred in the first 6 months and the child was classified as early faltering (early failure to thrive; FTT) (Fig. 1); the latter implied that it had occurred in the second 6 months, and the child was classified as late faltering (late failure to thrive; FTT) (Fig. 2). When the mean values for those trajectories were plotted (see Fig. 3) significant differences in standardised weight were found at 3, 6 and 9 months. The early FTT infants actually had higher birthweights than the others.