Twin-singleton influence on infant development: a national birth cohort study

While studying the development of twins, gestational age and birthweight were suggested as two of the major mediators to be considered. In addition, maternal age, maternal education, parental income and assisted reproductive technologies (ART) also should be considered as moderators. Thus the purpose of this study was to investigate the mediators and moderators of twin and singleton development.

Being a national birth cohort study, 21 648 infants were randomly selected and developmental measures were assessed at 6 and 18 months post partum. Children's development at 6 and 18 months were measured using the high reliable Taiwan Birth Cohort Study instrument, which measures children's development in four domains of gross motor, fine motor, language and social development. Additionally, maternal sociodemographics including maternal age, maternal education and parental income; children's characteristics including gender, birthweight, gestational age, single or multiple births, ART or natural conception information were also collected. These data were analysed using a three-step multiple linear regression analysis and further validated using structural equation modelling.

Parental sociodemographics, children characteristics and being twin all had effect on children's development. Additionally, ART and twin were mediators between maternal age and children's development. Mothers aged over 40 were more likely to choose ART, thus increases their likelihood of having twins. Additionally, mothers aged over 40 were more likely to give birth to premature or low-birthweight babies, regardless if they were twin or not. Twins had a higher prevalence of prematurity or low birthweight, which also affected their development in all four domains at 6 and 18 months. Thus prematurity and low birthweight were mediating factors between twin and children's development; with these two mediating factors controlled, there were no difference between twin and singleton development.

The conceptual construct of structural equation modelling showed these factors interacted and influenced children's development through multiple pathways. Medical intervention may facilitate children's development through prenatal growth and premature care.