Failure to Thrive (Poor Growth)

“Poor growth” is now the preferred term — it causes less parental distress than “failure to thrive”. Defined as weight:

• falling below the 3rd centile on ≥2 occasions, or
• crossing ≥2 centile lines over time.

These criteria are met by ~5% of infants at some point, most without a concerning trajectory — but the criteria will also miss some genuinely affected infants. Take clinical context into account (e.g. growth slowing over a winter of recurrent infections is less concerning than an isolated, fast drop-off); also consider ethnicity (e.g. children of Asian background are typically smaller than Caucasian children). International sources use a similar but not identical anthropometric definition — weight-for-age below the 5th percentile, a fall of more than 2 major percentile lines, or less than 80% of median weight-for-length — and increasingly also use a z-score (standard deviations below the mean weight-for-length), which is more precise for trending children whose weight sits well off the growth curve; mild, moderate and severe malnutrition correspond to z-scores of -1 to -1.9, -2 to -2.9, and below -3 respectively.

Typical weekly weight gain:

Age	Gain
0–3 months	150–200 g/week
3–6 months	100–150 g/week
6–9 months	80–90 g/week
9–12 months	70 g/week

Red flags — consider multidisciplinary involvement/admission

Signs of abuse or neglect; poor carer understanding (e.g. non-English speaking, intellectual disability); family vulnerability (drug/alcohol use, domestic violence, social isolation, no family support); signs of poor attachment; parental mental health issues; prior/current child protection involvement; did-not-attend or cancelled appointments; signs of dehydration, malnutrition or significant illness.

Causes

Usually multifactorial — no cause is found in ~80% of cases. Think in terms of caloric intake, absorption and expenditure. This maps onto the organic/non-organic framework used internationally: inadequate caloric intake from a non-organic cause is the most common etiology and is often multifactorial, while malabsorption and excessive-demand causes are more often organic; most real cases are mixed rather than purely one or the other.

• Inadequate intake: insufficient food offered; breastfeeding difficulties; physical causes (cleft palate, tongue-tie); persistent vomiting/GORD; chronic disease causing anorexia; restrictive diet (e.g. low-fat, vegan).
• Inadequate absorption: coeliac disease; chronic liver disease; pancreatic insufficiency (e.g. cystic fibrosis); cow’s milk protein intolerance; chronic diarrhoea/protein-losing enteropathy.
• Excessive utilisation: UTIs; chronic respiratory disease (asthma, bronchiectasis); congenital heart disease; diabetes; hyperthyroidism.
• Other medical: genetic disorders; inborn errors of metabolism.
• Psychosocial: poverty; parental depression; coercive feeding (mealtime battles); distractions at mealtimes; behavioural disorders; poor social support; neglect/abuse (4× higher risk of abuse in children with poor growth). A disinterested or depressed caregiver, or a mismatch between a child’s feeding demands and the caregiver’s capacity to meet them, can drive poor growth even without overt neglect; self-stimulatory behaviours (rocking, head banging) are sometimes noted in these children.

History

Antenatal/birth/postnatal history including growth parameters. Feeding history:

• Infants: breast (typically smaller) vs formula feeding, volumes/ frequency (can weigh before/after a breastfeed, or measure expressed volume), vomiting/GORD, timing of solid introduction.
• Toddlers: types/amounts of food and fluid (especially iron-containing foods), intake inside vs outside the home, mealtime battles, distraction, food refusal, milk volume, parental attitudes to food.

Also: medical history; developmental history (regression, known syndromes); family history (mid-parental height, short stature, parents’/siblings’ weight gain). Elimination history matters too — blood/mucus in stool, bulky/foul-smelling stools, or urinary symptoms point toward malabsorption or renal causes respectively.

Examination

Plot growth on the correct chart. General appearance — sick, irritable, lethargic? Dehydrated? Loss of subcutaneous fat (upper arms, thighs, buttocks)? Pallor? Inappropriate bruising? Jaundice? Observe feeding and parent–child interaction (ideally including a home visit). Dysmorphic features; developmental assessment; skin/hair/nails; goitre; ENT; lymphadenopathy; abdominal organomegaly; cardiorespiratory exam; finger-prick glucose.

Investigations

If otherwise healthy with no red flags on history/exam — no investigation is needed initially. If a specific diagnosis is suspected, investigate accordingly. For significant concern without a clear pointer, first-line: FBC, ESR/CRP, UEC, LFTs, iron studies, TFTs, blood glucose, urine MC+S, coeliac serology (if on gluten-containing solids), stool MC+S, stool fat globules/fatty-acid crystals. Reserve imaging (chest/abdominal X-ray, echocardiography) and second-line/subspecialist tests (sweat chloride, metabolic screen) for a positive history/exam pointer rather than routine screening.

Management

• Monitor weight — no more than weekly for 0–3 months, fortnightly after; excessive weighing can drive parental anxiety.
• Consider admission for illness, dehydration or suspected abuse; consider paediatrician review for persistent poor growth despite adequate intervention. Other indications for admission include the need to observe feeding directly, severe malnutrition, or failed outpatient management.
• Lactation consultant or early childhood nurse support; dietician for older children. Catch-up growth typically needs extra energy intake — roughly 150% of the child’s normal caloric requirement — achieved by increasing feed volume/frequency first, then caloric density (standard infant formula/breastmilk is ~20 kcal/oz; most infants tolerate fortification to 22–24 kcal/oz, occasionally higher) if volume increases alone aren’t enough. For children with significant malnutrition, reintroduce feeds cautiously with monitoring for refeeding syndrome (hypophosphataemia, hypomagnesaemia, hypokalaemia).
• Assertive follow-up — act immediately on missed appointments; refer to child protection if concerned. Approach caregivers as partners in the assessment rather than as suspects by default — most non-organic poor growth reflects a lack of resources, knowledge or support rather than deliberate neglect, though abuse must still be considered and acted on when red flags are present.

Prognosis

Children who develop poor growth before age 1 (especially before 6 months, when postnatal brain growth is fastest) carry the highest risk of later cognitive/developmental delay, so earlier identification and intervention matters more than the specific cause. With appropriate feeding support, most children over age 1 with non-organic poor growth reach a stable weight above the 3rd centile; outcomes for organic causes track the underlying condition’s own prognosis once it’s treated.