Genetic and Environmental Influences on Developmental Milestones and Movement: Results From the Gemini Cohort Study.

<bold>Purpose: </bold>Variability in the timing of infant developmental milestones is poorly understood. We used a twin analysis to estimate genetic and environmental influences on motor development and activity levels in infancy.<bold>Method: </bold>Data were from the Gemini Study, a twin birth cohort of 2,402 families with twins born in the United Kingdom in 2007. Parents reported motor activity level for each of the twins at age 3 months using the Revised Infant Behavior Rating Scale and reported the ages at which they first sat unsupported, crawled, and walked unaided.<bold>Results: </bold>Activity level at 3 months and ages when first sitting and crawling were about equally influenced by the shared family environment (45%-54%) and genes (45%-48%). Genetic influences dominated for age when children took their first independent steps (84%).<bold>Conclusion: </bold>Aspects of the shared family environment appear to be important influences on motor activity levels and early milestones, although the timing of walking may have a stronger genetic influence. Further research to identify the specific environmental and genetic factors that promote early activity may be important for longer-term health outcomes. [ABSTRACT FROM AUTHOR]