Follow-up study of physical growth of monozygous twins with discordant within-pair birth weights.

In an attempt to determine the future growth of intrauterine growth-retarded babies, 14 pairs of monozygous twins, showing within-pair birth weight differences of 11% to 48% (median 28%), were followed prospectively for 3 to 9 years. Weight, length, and head circumference were measured biannually for the first 3 years of life and thereafter annually. At birth, the median within-pair percentage differences of weight (28%), length (6.2%), and head circumference (5.2%) were all significant (P less than .01). At 12, 24, and 30 months of age, these median within-pair percentage differences became insignificant for head circumference, length, and weight, respectively. The most rapid catch-up growth of the lighter twins occurred during the first 2 years of life. Analysis of individual pairs, however, showed suboptimal growth in both members of two pairs, although growth became concordant. In four other pairs, the lighter birth weight member remained growth retarded in comparison with the cotwin and showed within-pair differences in weight varying between 8% and 19%, in height between 1% and 6%, and in head circumference between 1.9% and 7%. All children with birth weights above the tenth percentile grew up normally, regardless of the extent of within-pair birth weight differences, with one exception. This exception was a pair in whom there were within-pair differences in child rearing. In the six pairs in which the smaller twin's birth weight was below the tenth percentile, only three pairs showed normal growth. In these six pairs a normal ponderal index in the lighter twin members was associated with poorer growth than a low ponderal index.(ABSTRACT TRUNCATED AT 250 WORDS)