Human Development · Paper A
Language milestones
Speech and language development follows a predictable timeline, and specific missed milestones act as clear red flags for referral. For example, imagine a child with no words at all by 18 months. In practice, knowing the exact ages lets you recognise a genuine delay rather than normal variation.
Cooing begins around 6 weeks to 3 months, and babbling around 6 months. A first word typically appears around 12 months. By 18 months, children usually have 10 to 20, up to 50, words, and begin pointing. By 2 years, children combine two-word phrases and have over 50 words. By 3 years, they use three-word sentences and have over 200 words. By 4 years, they use complex sentences. No words at all by 18 months, or no two-word phrases by 2 years, are recognised warning signs warranting a hearing assessment and referral. For example, imagine a 2-year-old who still speaks only single words with no two-word combinations. In practice, this means that specific gap is the warning sign that triggers referral.
Language milestones are the expected ages at which typically developing children reach specific speech and language skills, and they matter because missed milestones are among the clearest, most actionable red flags in child development. Language delay can be the first visible sign of several different underlying problems, so recognising the normal timeline, and knowing exactly when it has been breached, is a core skill for any doctor who might see children. For example, imagine a worried parent bringing in their 20-month-old who has not yet said a single recognisable word. Knowing the expected timeline lets you judge immediately whether this is a genuine concern or within normal variation.
A language milestone is simply the age by which most typically developing children can be expected to have reached a specific speech or language skill, from early sounds through to full sentences. Milestones give clinicians a shared, standardised timeline to compare an individual child against.
The caveat that trips up beginners is treating "language delay" as a single diagnosis. It is not: it is a presenting complaint that sits at a crossroads of several different possible underlying problems. The same complaint, "my child isn't talking," can turn out to reflect isolated speech-and-language delay, global developmental delay (delay across multiple developmental domains, not just language), autism spectrum disorder, a simple treatable hearing problem, or, less commonly, selective (a child who can speak normally but does not speak in specific settings, such as school, due to anxiety). The assessment that follows a missed milestone exists specifically to sort between these possibilities, not simply to confirm delay exists.
It is worth noting, as a general rule about early development, that receptive language (understanding what is said) develops ahead of expressive language (producing speech), a child usually understands far more than they can say at any given age. This is why assessment of a "late talker" always tests understanding as well as speech output: a child with good receptive skills but poor expressive skills is a very different clinical picture from a child who struggles to understand language at all.
Cooing (soft vowel-like sounds) begins around 6 weeks to 3 months of age. Babbling (repeated consonant-vowel sounds, such as "ba-ba" or "da-da") begins around 6 months. A child's first true word typically appears around 12 months. By 18 months, most children have a vocabulary of about 10 to 20, up to 50, words, and begin pointing to show interest or make requests. By 2 years, children are combining words into two-word phrases and typically have more than 50 words in total. By 3 years, three-word sentences appear and vocabulary passes 200 words. By 4 years, children are using complex, grammatically fuller sentences.
Once a delay is confirmed, the real diagnostic work is telling apart its possible causes, because each looks similar at first glance but needs a different pathway. Isolated speech-and-language delay affects communication specifically, with other developmental domains, motor skills, social skills, play, progressing normally. Global developmental delay affects language alongside multiple other domains, such as motor and cognitive skills, and usually points toward a broader underlying condition. Autism spectrum disorder can also present with language delay, but is distinguished by accompanying difficulties with social communication and restricted or repetitive behaviours. Hearing loss can mimic every one of the above, since a child who cannot hear well cannot learn spoken language normally, which is exactly why it must always be checked first. Selective is a further look-alike worth knowing: unlike true language delay, the child's language skills are normal, but speech is absent in specific anxiety-provoking settings while present elsewhere, such as at home.
Whenever a child presents with delayed language, the first and mandatory investigation is a hearing (audiology) assessment, because hearing loss is a common and treatable cause of apparent language delay that can be missed if clinicians jump straight to more complex explanations. Suppose a child has no words by 18 months and their hearing assessment comes back entirely normal. That normal result shifts the diagnostic picture firmly toward other causes and justifies moving on to broader developmental screening, which uses structured developmental history and direct observation of play, social interaction, and communication to place the delay into one of the categories above. Certain missed milestones are recognised red flags that should prompt this referral rather than reassurance and waiting: no babbling by around 12 months, no single words by around 18 months, no two-word phrases by around 2 years, and, most importantly, any loss of previously acquired language, since regression can signal a serious underlying neurological or developmental condition and should never simply be watched.
Management differs by pathway, which is exactly why getting the diagnosis right matters. Isolated speech-and-language delay is typically managed with speech-and-language therapy (SLT) input and ongoing monitoring of progress against the milestone timeline. Global developmental delay or suspected autism spectrum disorder triggers a fuller multidisciplinary assessment and a management plan tailored to the wider diagnosis, often involving paediatrics, psychology, and educational support alongside speech-and-language therapy. Confirmed hearing loss is managed by addressing the hearing problem itself, for example with hearing aids, after which language often catches up with appropriate support. UK guidance, including NICE recommendations on identifying developmental concerns in children, supports early referral over a "wait and see" approach once a red-flag threshold is crossed, because earlier intervention is associated with better outcomes.
The single fact most likely to appear in a vignette is that any regression, a child who used to speak and then stops or loses previously acquired words, is always a red flag regardless of age, and should never be reassured away, unlike a simple missed milestone which can sometimes fall within normal variation. The second high-yield point is the order of investigation: hearing assessment always comes before assuming a neurodevelopmental cause.
Language milestones run from cooing at 6 weeks through babbling at 6 months, first words at 12 months, and two-word phrases at 2 years, and any missed threshold, especially no words by 18 months, no two-word phrases by 2 years, or any regression, should prompt a hearing test first, then broader developmental assessment to sort isolated delay from global delay, autism, or hearing loss.