Developmental Milestones & Behavior
Four streams, a handful of red flags, and when to worry
Framework: four streams and global delay
Four domains. Screen development at every well-child visit in four streams: gross motor (sit, crawl, walk, run), fine motor (grasp, pincer, draw), language (babble, words, sentences), and social/emotional (smile, play, interaction). A lag in one stream is an isolated delay; lags in two or more are global developmental delay. Language and social delays are the most important to catch early, because early intervention changes outcomes.
Global delay workup. Evaluate with chromosomal microarray and fragile X testing (fragile X is the most common inherited cause of intellectual disability; Down syndrome is the most common genetic cause overall), plus metabolic studies when indicated and formal hearing and vision testing. Exome sequencing is now also an accepted first-tier test (ACMG 2021). Any language delay needs an audiology evaluation.
Milestones by age
| Age | Gross motor | Fine motor | Language | Social / cognitive |
|---|---|---|---|---|
| 2 mo | Lifts head when prone | — | Coos | Social smile |
| 4 mo | Rolls front to back; no head lag | Reaches for objects | Laughs | — |
| 6 mo | Sits in tripod; rolls both ways | Transfers objects hand to hand | Babbles | Stranger anxiety begins |
| 9 mo | Sits independently, crawls, pulls to stand | Pincer grasp develops | — | Object permanence |
| 12 mo | First steps | Mature pincer | 1–3 words plus specific mama/dada | Separation anxiety (peaks 12–18 mo) |
| 18 mo | Runs | Scribbles, uses a spoon | 10–25 words; points to body parts | — |
| 2 yr | Up and down stairs, kicks a ball | Builds a tower, copies a line | 2-word phrases, ~50 words, two-step commands | Parallel play |
| 3 yr | Rides a tricycle | Copies a circle | 3-word sentences | — |
| 4 yr | Hops on one foot | Copies a cross and a square | Tells stories | — |
| 5 yr | Skips | Copies a triangle | — | — |
Object permanence (about 9 months) is why peek-a-boo delights infants: they now know the hidden face still exists.
Language rule of numbers. Words per phrase roughly equal age in years through age 3: about 1 word at 1 year, 2-word phrases (and roughly 50 words) at 2, 3-word sentences at 3. Intelligibility to strangers follows quarters of age.
Red flags and regression
Escalate for:
- No social smile by 2 months
- No babble or reciprocal gestures by 9–12 months
- No single words by 16–18 months
- Not walking by 18 months
- No 2-word phrases by 24 months
- Hand preference before 12 months, which suggests contralateral weakness such as cerebral palsy
- Persistence of primitive reflexes (Moro, grasp) well beyond early infancy, another sign of CNS injury
Loss of previously acquired skills at any age is never normal and mandates workup for a neurodegenerative or metabolic disorder.
Rett syndrome. An MECP2 mutation in girls causes regression at 6–18 months with loss of purposeful hand use, stereotyped hand-wringing, loss of speech, and acquired microcephaly (decelerating head growth).
Autism spectrum disorder and ADHD
| Feature | Autism spectrum disorder | ADHD |
|---|---|---|
| Core features | Persistent deficits in social communication plus restricted, repetitive behaviors and interests | ≥6 inattentive and/or hyperactive-impulsive symptoms |
| Timing and setting | Early developmental period | Before age 12, in ≥2 settings (home and school), for ≥6 months, with impairment |
| Sex | Boys about 4:1 | More common in boys |
| Screening or assessment | M-CHAT at 18 and 24 months | Rating scales from parents and teachers |
| Treatment | Early behavioral intervention (applied behavior analysis) | Stimulant (methylphenidate or amphetamine); behavior therapy first under age 6 |
Autism clues. No pointing to show interest, poor eye contact, absent words, and lining up toys at 20 months call for M-CHAT screening and referral, not reassurance. Fragile X and tuberous sclerosis are the classic associated genetic conditions, so a diagnosed child gets genetic testing and a hearing evaluation. Antipsychotics such as risperidone can target severe irritability but do not treat the core deficits.
ADHD treatment. Stimulants are first-line in school-age children; atomoxetine and α2 agonists (clonidine, guanfacine) are alternatives. Monitor appetite, sleep, and growth on stimulants.
Failure to thrive and anticipatory guidance
Failure to thrive is weight below the 2nd–5th percentile or a fall across two major percentile lines. By far the most common cause is inadequate caloric intake (nonorganic or psychosocial), not organic disease, so start with a diet history and an observed feeding rather than an exhaustive lab workup. Consider neglect, and admit when malnutrition is severe or outpatient management fails.
Reading the growth chart. Undernutrition lowers weight first, then length, and head circumference last. A small head from the start suggests an intrauterine or genetic cause, and height falling before weight suggests an endocrine cause.
Anticipatory guidance addresses the next stage's risks before they arrive:
- Infants: back to sleep on a firm surface without soft bedding; rear-facing car seat as long as the seat's height and weight limits allow.
- Toddlers: choking hazards, no baby walkers, water and poison safety.
- Children on wheels: a helmet every time.
- Discipline: time-out of roughly one minute per year of age.