At 3 months of age, which developmental milestone is most likely delayed in an infant who does not smile or coo?

At 3 months of age, the absence of smiling or cooing is primarily indicative of delays in social and verbal development. At this stage, infants typically begin to engage socially by smiling at people and making cooing sounds as a form of communication. Smiling is often one of the first indicators of social engagement, while cooing represents the beginnings of verbal communication.

While motor development is important and includes reflexive movements that develop early in life, it does not directly relate to social interactions or communication. The ability to smile and coo reflects responsiveness to social stimuli and the development of early verbal skills, making social and verbal development interconnected. Thus, the most significant concern, when observing that an infant at this age does not smile or engage through sounds, is that their social and verbal cognitive development is not progressing as expected for their age.

Overall, the lack of social smiles and cooing points to delays particularly in areas of social and verbal development rather than motor or all areas of development.