Growth and Development of Twins Dissimilar in Size at Birth

Serial observations on nine pairs of monozygous twins who differed in birth weight on the average of 36 per cent indicated that the undersized members continue to be inferior in both growth and intelligence into adult life. Some equalization occurs in school performance, as opposed to test scores, suggesting that the smaller twin has made a compensatory effort. The twin who was smaller at birth remained the smaller in height, weight and head circumference. In the eight pairs who have reached maturity, mean differences continue to have the same order of magnitude as in previous comparisons, but head circumference is less affected than height and weight. Although monozygotic pairs are a useful model for study of the effect of fetal undergrowth on later development, interpretation must be done cautiously in view of the complex interaction of placental and environmental factors. (N Engl J Med 289:937–940, 1973)