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Temperament differences between cotwins at least 15% discordant for birth-weight were compared for a group of 66 preterm twin pairs and a group of 70 fullterm twin pairs, assessed in the neonatal period. The assessment focused on irritability, resistance to soothing, activity, reactivity, and reinforcement value. For the most part, significant differences were not obtained between discordant preterm cotwins. Previous research  had demonstrated that the larger fullterm twin of discordant pairs was more irritable, more difficult to soothe, more active while awake and during sleep, less reactive to visual and auditory stimuli, and less reinforcing to the examiner than the smaller cotwin. It was concluded that, for the behavioral variables measured, the fullterm group is at higher risk than the preterm group, and that both the lighter and the heavier discordant twins displayed signs of early behavioral risk. The findings suggested that the continued gestation for cotwins discordant in weight may be a risk period for early temperament development of the twins.
Neonatal sleep behaviors and behavioral state cycling were observed for 20 pairs of same-sex, fullterm twins in which one twin of the pair was appropriate-for-gestational-age (AGA) and the other twin was small-for-gestational-age (SGA). Time-sampling recordings were made in active sleep of number and vigor of limb movements, body and head movements, and mouth movements. No group differences were observed for time spent in first active sleep, first quiet sleep, or length of first sleep cycle. Examination of specific behaviors indicated a significantly higher incidence of vigorous limb movements and right hand-to-mouth movements, with a trend for more small limb movements and left hand-to-face movements, for AGA twins when compared with SGA twins. SGA twins had significantly more spontaneous smiles and a trend for more spontaneous startles than AGA twins. A stepwise discriminant analysis indicated that a composite of the variables smile, large limb movement, startle, and left hand-to-face significantly discriminated between the two groups, with 90% correct classification of the AGA twins and 75% correct classification of the SGA twins. The results demonstrated the utility of evaluating specific sleep behaviors, rather than state cycling only, to describe differences in neonatal sleep characteristics between AGA and SGA twins.
Infants from 22 pairs of appropriate-for-gestational-age/small-for-gestational-age (AGA/SGA) same-sex twins were assessed for temperament stability between the neonatal period and 30 months of age. The evaluation of neonatal temperament included observers' ratings of irritability, resistance to soothing, activity level while awake, activity level during sleep, reactivity, and reinforcement value. At 6, 9, 12, 18, 24, and 30 months mothers rated their infants' temperament on standardized questionnaires which yielded nine temperament categories: activity level, rhythmicity, approach or withdrawal, adaptability, intensity of reaction, quality of mood, attention span, distractibility, and threshold of responsiveness. The Bayley Scales of Infant Development were administered at 6, 9, 12, 18, 24, and 30 months to assess mental development. The results of longitudinal correlational analyses indicated that, for the AGA infants, there was a pattern of significant predictive relations between the neonatal ratings of temperament and maternal ratings of temperament at 6, 9, 12, 18, 24, and 30 months. The SGA infants did not demonstrate similar behavioral stability from the lying-in period up to 30 months of age. Furthermore, for AGA infants there was a consistent trend for both reactivity and neonatal activity during sleep to predict mental development scores between 6 and 30 months. A similar pattern was not observed for the SGA cotwins. The results indicated that temperament stability is an additional area of risk for SGA twins, and that the developmental function of the underlying processes in the neonatal measures is different for AGA and SGA infants.
The genetic contribution to temperament was assessed during the neonatal period in 172 newborn infants from 47 pairs of monozygotic (MZ) and 39 pairs of same-sex dizygotic (DZ) twins. Zygosity was not related to the neonatal temperament variables. Examination of the scores for twin concordance indicated significant within-pair concordance in temperament ratings, but MZ twins were not more like each other than DZ twins. The results suggested that neonatal temperament was influenced by environmental factors. Further analyses indicated that neonatal temperament was influenced by perinatal variables such as birth weight, 1- and 5-minute Apgar scores, and number of days spent in the hospital. It was concluded that there was no clear pattern of genetic influence on neonatal temperament, and that nonshared environmental factors were an important source of variance for temperament during this period.
Ten pairs of full-term and 8 pairs of preterm twins from combined vaginal and Cesarean section deliveries were evaluated on measures of neonatal temperament, developmental status, and integrity to determine if the Cesarean-delivered infant was compromised relative to its vaginally-delivered twin. There were no significant differences, within groups, between the vaginal and Cesarean section infants on measures of risk and developmental status. Analyses of variance of paired comparisons performed on the temperament measures indicated that, for the full-term group, there were no significant differences in ratings between vaginally and Cesarean-delivered infants. In contrast, preterm infants delivered by Cesarean section were more active during sleep than their vaginally-delivered cotwins. This finding, together with previous findings demonstrating a relation between this measure and temperament at 9, 18, and 24 months of age, suggested that the preterm Cesarean-delivered infant may be at risk in this area when compared with its twin. In the main, however, the results demonstrated that infants born by Cesarean section following vaginal delivery of their twins were not more compromised than their twin siblings.
The predictive relationship between neonatal behavior and temperament at 6 months was assessed in 62 pairs of twins. The neonatal assessment evaluated irritability, resistance to soothing, reactivity, and activity level across various activities. Temperament at 6 months was appraised by a laboratory assessment, ratings on the Infant Behavior Record, and a questionnaire completed by the mother. Examination for temperament consistency from the neonatal period to 6 months indicated that (a) neonates rated by an examiner as irritable and difficult to soothe were reported by the mother to be more negative, irritable, and slower to adapt at 6 months; (b) neonates rated as more irritable and active were more attentive and responsive during mental testing at 6 months; (c) there was no relationship between the neonatal measures and the laboratory ratings, possibly reflecting methodological problems influenced by maturation and vegetative processes at 6 months. Analyses for patterns of concordance within the various data sets for MZ and DZ twins indicated that there was no pattern of differential concordance favoring MZ twins for the temperament measures at birth or at 6 months. It was concluded that a modest predictive relationship between temperament variables had been demonstrated from birth to 6 months, but no specific genetic effects were detected at these early ages.
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