Objective: To investigate how the degree of prematurity interacts with genetic and environmental influences in their effect on verbal and nonverbal cognitive development.
Study design: The target sample consisted of more than 2000 pairs of twins born in England and Wales in 1994. At 24 months, measures of verbal and non-verbal cognitive development were obtained from the twins' parents. The sample was divided into 3 groups according to degree of prematurity: very preterm or high-risk (<32 weeks), moderately preterm or medium-risk (32-33 weeks), and mildly preterm/term or low-risk (>34 weeks). Quantitative genetic analyses were used to assess the contributions of genetic and environmental influences on vocabulary and cognitive development.
Results: The results indicated gene-environment interactions. For the high-risk group, genetic effects on both verbal and non-verbal cognitive ability were completely overshadowed by shared environmental factors, whereas for both medium- and low-risk groups, additive genetic effects explained 18% to 33% of the variance.
Conclusions: Our findings indicate that genetic factors are not responsible for cognitive outcomes of very preterm infants and suggest that early environmental influences appear to affect verbal and non-verbal cognitive development at 2 years of age.