Human Development · Paper A
Developmental milestones
Developmental milestones give expected ages for specific motor and social skills, and knowing them lets you recognise when a child is meaningfully behind rather than simply developing at their own pace. A child who is not yet walking at 18 months has crossed a specific, well-defined threshold, not a vague sense of 'slowness', that should prompt referral. Milestones are checked against fixed ages precisely so that delay can be recognised early and consistently.
By 6 weeks, an infant shows a social smile. By 6-7 months, they sit with support, use a palmar grasp, and transfer objects between hands. By 9 months, they sit unsupported, crawl, pull to stand, and show stranger anxiety. By 12 months, they walk unsupported, say first words, use a pincer grip, and wave bye-bye. By 18 months, they walk well, run, and use 10-20 words. By 2 years, they use two-word phrases and run confidently, and by 3 years, they climb stairs and talk in full sentences. A child who is not walking by 18 months and also has no words by 18 months shows two independent red flags, e.g. gross-motor and language delay together, that should prompt referral rather than being watched further. Two independent red flags together strengthen the case for referral more than either alone.
A fresh doctor might rely on a general sense of whether a child 'seems behind'. That impression is unreliable; developmental milestones matter precisely because they replace impression with a fixed, checkable age. Hold the phrase FIXED AGE, NOT IMPRESSION through every section below.
DEVELOPMENTAL MILESTONES are expected ages by which a child typically achieves specific motor, sensory, language and social skills, grouped into four DOMAINS (separate skill categories tracked independently): gross motor, fine motor and vision, hearing/speech/language, and social/behavioural.
A common mistake is referring a child for a single, mildly late milestone in isolation. A genuine RED FLAG (for example, not walking by 18 months, or no single words by 18 months) should prompt referral, but an isolated, slightly late milestone in an otherwise typically developing child is far less concerning. Two independent red flags together, e.g. gross-motor delay plus language delay, strengthen the case for referral more than either alone.
A child not yet walking at 18 months has crossed a specific, well-defined threshold, the standard gross-motor red-flag age, not a vague sense of 'slowness'. Milestones exist as fixed ages precisely so that delay can be recognised early and consistently, rather than relying on subjective parental or clinical impression alone.
By 6-8 months, an infant sits with support and later unsupported; by 9 months, they pull to stand and crawl; by 12-15 months, they walk unsupported. Fine motor and vision progresses from a palmar grasp (whole-hand grip) around 6 months, to transferring objects between hands, to a mature pincer grip (thumb-finger grip) by around 9-12 months, letting a child pick up small objects deliberately.
By 6 weeks, an infant shows a social smile. By 9 months, stranger anxiety typically emerges. First words appear around 12 months, two-word phrases by around 2 years, and full sentences by around 3 years. Socially, waving bye-bye appears around 12 months, and by 3 years most children engage in interactive, imaginative play with peers rather than purely solitary play.
A milestone DELAY should not be confused with DEVELOPMENTAL REGRESSION (losing a previously achieved skill), which is a red flag in its own right and prompts a different, more urgent work-up (for example, for a neurodegenerative or metabolic condition) than simple delay in reaching a milestone on time. Global developmental delay (multiple domains affected together) is also distinct from an isolated single-domain delay, e.g. speech delay alone, which carries a narrower differential.
Health visitors and the NICE-aligned Healthy Child Programme use these milestone ages as the backbone of routine developmental screening in the UK, checking each domain against its expected age at standard review points, e.g. the newborn check, the 6-8 week check, and later reviews around one and two to two-and-a-half years. Some settings supplement this with a structured, parent-completed screening tool, e.g. the Ages and Stages Questionnaire, to make the check more systematic than a purely clinical impression. A confirmed red flag or developmental regression at any of these checkpoints warrants further assessment and investigation, e.g. hearing tests, growth and vision checks, and specialist paediatric referral, rather than a purely 'watch and wait' approach.
Management of a confirmed delay depends entirely on the underlying cause once identified, ranging from targeted therapy input to management of an underlying medical condition. An isolated language delay is typically managed with speech and language therapy; an isolated gross motor delay may prompt physiotherapy input and a check for an underlying neuromuscular cause; global developmental delay, affecting multiple domains together, usually warrants referral to a community paediatrician or child development service for a broader work-up, which can include hearing and vision assessment, growth monitoring, and sometimes genetic or metabolic investigation if regression or additional red flags are present. Throughout, the clinician should also keep safeguarding in mind if the pattern of delay doesn't fit a straightforward developmental or medical explanation.
Vignettes often describe a single, mildly late milestone in an otherwise well child and expect recognition that this alone does not meet red-flag criteria for referral. Another trap describes a child who LOSES a previously achieved skill, expecting recognition of regression, a more urgent flag than simple delay, rather than simple reassurance. A further trap tests whether you know the specific ages that define red flags, e.g. not walking by 18 months, rather than a vague sense of 'behind'.
Learn the fixed ages, not the vague impression: a genuine red flag or any regression earns referral through the Healthy Child Programme, a single mildly late milestone alone usually doesn't.