Based on the progressive trajectories of typically developing children, tools for tracking the development of children with complex disabilities (e.g., the Carolina Curriculum) anticipate that they will follow similar paths, albeit at a slower pace. When children’s development not only stagnates but appears to regress, it can therefore be a challenge both to parents and to the professionals in their clinical partnership. Taking a biopsychosocial approach, we will present data from 30 parents of children with complex developmental delays who completed a developmental questionnaire (the ABASII) at six monthly intervals four times over the preschool period. Results suggest that while most children have a consistent forward trajectory as babies, as they enter the pre-school period, their development becomes increasingly characterised by temporary loss of/imbalance between skills until they are at least five years old when the trajectory begins to return to a more coordinated forward path. These data are in stark contrast to those from 28 typically developing children from whom the same data were collected on two occasions, a year apart. None of this control group showed any backtracking in skill acquisition at any age. This data suggest (a) that all professionals including early childhood teachers should understand and anticipate the uneven development of children with disabilities and (b) that if children with complex disabilities are to be encouraged to move on to school before the age of six, then primary teachers need to be better prepared to support such a fragile stage of the children’s development.