Child development is a complex, non-linear process affected by multiple factors. In addition, there is a large degree of individual variation.
Developmental delay is present when a child does not reach developmental milestones at the expected age (with adequate leeway for the broad variation among normal children). Although delay may result primarily from a biological factor such as a chromosomal disorder, or an environmental factor such as maternal depression, the principal model for the causes of developmental delay is a ‘transactional’ one. The process of development is viewed as a transaction between the child and the environment, in which each can have profound effects on the other.
About 15% of children have developmental delay. Many, however, are not detected before commencing school, mainly because the disabilities are mild or because they relate to tasks only then attempted by the child. Of the 15%, a much smaller proportion has more severe disability. This group is more likely to present earlier because of the severity of their problems, because there is more often an associated medical condition, and because a number of them (such as those with extreme prematurity) are picked up by at-risk screening programs.