Temperament development to 30 months of age in discordant twin pairs.

Twins within pairs often have different weights at birth. A difference of 15% or greater is defined as discordance for weight and is considered to place one or both infants at risk. Temperature differences had been found in the neonatal period for fullterm discordant cotwins, but not for preterm discordant cotwins, suggesting that continued gestation for discordant twins was a risk variable for early behavior. 30 pairs of fullterm and 17 pairs of preterm discordant pairs were followed at 6, 9, 12, 18, 24, and 30 months of age. Group differences were observed for the longitudinal maintenance of cotwin discordance in physical measures, with preterm cotwins becoming more like each other. In laboratory assessments, temperament differences no longer were observed between the larger and smaller cotwins. Questionnaires indicated that mothers generally did not differentiate between their larger and smaller cotwin children in temperament ratings, except for ratings of mood for the fullterm pairs. Thus, emotionality was the only temperament dimension that differentiated between the fullterm discordant twins both in the neonatal period and at later ages. In the main, it was concluded that the fullterm discordant twins overcame the adverse in-utero influences on early behavioral development.