Poor Weight Gain: Causes in Babies and Children

What is normal weight gain?

Babies often lose 5 to 7% of their birth weight in the first few days, and up to 10% can be normal in breastfed babies. Most are back to birth weight by 10 to 14 days. After that, typical gains are:

Most babies double their birth weight by 4 to 5 months and triple it by 12 months. Growth is assessed on a chart rather than week by week: children normally follow their own centile line. Small babies born to small parents can be healthy on a low centile. What matters is the trend.

Causes of poor weight gain

Not taking in enough calories (most common)

• Breastfeeding difficulties: poor latch, infrequent feeds, low milk supply, or tongue-tie.
• Formula preparation errors: too much water, or too few feeds.
• Feeding difficulties: slow or uncoordinated sucking, prematurity, cleft palate, or neurological conditions.
• Picky eating and food refusal: in toddlers; also sensory feeding difficulties and avoidant/restrictive food intake disorder.
• Excess juice or milk in toddlers filling them up.
• Social and family factors: stress, food insecurity, parental depression or difficulties with routines.

Losing calories

• Reflux disease: spitting up is normal, but frequent vomiting with feeding refusal and poor weight gain needs review.
• Cow's milk protein allergy: vomiting, diarrhea, blood or mucus in the stool, eczema and fussiness.
• Pyloric stenosis: forceful projectile vomiting at 2 to 8 weeks.
• Chronic diarrhea: from infections or toddler's diarrhea.

Not absorbing nutrients

• Celiac disease: after gluten is introduced; bloating, diarrhea, irritability, poor growth.
• Cystic fibrosis: greasy, foul-smelling stools, chest infections, salty skin. US newborn screening detects most cases.
• Inflammatory bowel disease in older children.

Needing extra energy

• Congenital heart defect: tiring, sweating and breathlessness during feeds.
• Chronic lung disease, prematurity-related lung problems.
• Chronic infections: urinary tract infections, HIV or tuberculosis.
• Overactive thyroid (rare).

Other medical causes

Kidney disease, metabolic and genetic conditions, and hormone problems can all affect growth. These usually come with other signs.

How doctors assess poor weight gain

1. Accurate measurement: weight, length or height, and head circumference plotted on the WHO chart (under 2) or CDC chart (2 and over).
2. Feeding history: how often, how much, how long feeds take, and what happens during and after.
3. Watching a feed: breastfeeding or bottle feeding observed by a nurse, lactation consultant or pediatrician.
4. Examination: for signs of illness, dysmorphic features or developmental delay.
5. Tests if needed: urine test for infection, blood count, celiac antibodies, kidney, liver and thyroid function, stool tests, and sometimes a sweat test or genetic tests.

In most children, a careful feeding assessment explains the problem and extensive tests are not needed.

Treatment and support

• Breastfeeding support: a lactation consultant can fix latch problems, increase feed frequency, and advise on expressing or supplementation when needed.
• Correct formula preparation: always follow the instructions on the label.
• Extra calories for toddlers: offer 3 meals and 2 to 3 snacks at regular times, add healthy fats (butter, olive oil, full-fat dairy, nut butters for children over age-appropriate risk), and limit juice and milk to recommended amounts.
• Calm mealtimes: eat together, avoid pressure or force feeding, and limit meals to about 30 minutes.
• Treat underlying conditions: for example removing cow's milk protein or gluten, treating reflux disease, or managing heart and lung disease.
• Specialist help: dietitians, speech and feeding therapists and pediatric gastroenterologists for persistent problems.

Weight is usually rechecked every 1 to 4 weeks until the child is back on track.

Poor weight gain in adults

In adults, difficulty gaining weight or being underweight (a BMI below 18.5) is often due to low intake, high activity or a fast metabolism, but it can also reflect overactive thyroid, diabetes, celiac disease, inflammatory bowel disease, depression, eating disorders or cancer. Unintended weight loss of more than 5% in 6 to 12 months should always be checked by a doctor.

Frequently asked questions

How do I know if my baby is not gaining enough weight?

Signs include not regaining birth weight by 2 weeks, fewer than 6 wet diapers a day after the first week, long or unsatisfied feeds, and weight falling across centile lines on the growth chart. Regular weight checks with your pediatrician are the most reliable way to tell.

What is failure to thrive?

Failure to thrive, now often called faltering growth, means a child's weight gain is much slower than expected for their age and sex. It is a description, not a diagnosis. The cause is usually low calorie intake, but medical conditions are checked when needed.

Can a breastfed baby have poor weight gain?

Yes, though breastfed babies normally gain a little more slowly after 3 to 4 months than formula-fed babies. Poor gain can come from latch problems, infrequent feeds, tongue-tie or low supply. A lactation consultant can assess a feed and help.

Will my child catch up after poor weight gain?

Most children catch up once the cause is found and calorie intake improves. Weight usually recovers first, followed by length or height. Children with chronic conditions may need longer-term support. Regular growth checks help confirm catch-up.

What foods help a toddler gain weight?

Offer energy-dense foods such as full-fat yogurt, cheese, eggs, avocado, nut butters, olive oil on vegetables and pasta, and smoothies. Serve regular meals and snacks, and limit juice and milk so they do not fill your child up. A pediatric dietitian can give a personalized plan.