Growth in the first four years: I. The relative effects of gender and weight for gestational age at birth.

The growth of 238 small-for-dates (SFD), 246 average-for-dates (AFD) and 241 large-for-dates (LFD) babies was assessed at regular intervals from birth to four years. At all ages, and for both sexes, the mean weight, length or height, and head circumference values for the two extreme groups differed significantly from the AFD group (P less than 0.001). During the first six months actual and relative growth was highest in the SFD group and lowest in the LFD group. With the exception of length in the LFD group it was higher for boys than for girls. Thereafter differences in actual values were slight, while the proportional increases for the three groups were almost identical. Differences from the AFD mixed-sex mean value were plotted for each group and sex combination. Stabilization of the mean trajectories in the two extreme groups occurred at six months for weight and head circumference, but not until 12 months for length. For each child a straight line was fitted on a log scale to these differences from six months (12 months for length) to four years, and analyses of variance of the slopes and intercepts obtained. The sex difference for weight and height gradually decreased, but for head circumference held constant. Estimates of the difference in intercepts of the birthweight groups and sexes showed that within the LFD group the difference in height between boys and girls was much less than in the other two groups. The sex difference for head circumference was slightly larger than the difference between SFD and AFD groups, and twice the difference between the LFD and AFD groups. At the age of four years the mean value for AFD boys was higher than LFD girls (P less than 0.01), and those for SFD boys and AFD girls were almost identical.