Reasons why your child could be underweight

Author : Dr Namratha Upadhya 

Pediatric Endocrinologist, Aster RV Hospital 

Underweight is a term often used casually to describe children who appear thinly built. In practice, a child is considered to be underweight if his or her Body mass index (BMI) is less than the 5th percentile for age and sex, when plotted on a BMI chart. There are several reasons why a child could underweight such as:


Inadequate calories due to poor quality of diet, lack of awareness about healthy eating or any disease causing poor dietary intake or absorption of nutrients, or increased demand for nutrients can result in the child becoming underweight. Most often, linear growth, namely height is also affected resulting in the child also being short.  Malnourished children may appear very thin, with loss of fat over the cheeks, arms, buttocks, may appear dull and less active compared to healthy children. They may have symptoms or signs of an underlying disease such as or difficulty in feeding chronic diarrhea, vomiting, protruding belly, a chronic cough, wheezing, a hormonal disorder such affecting thyroid or adrenal glands, diabetes or a neurological disorder causing difficulties in feeding, swallowing, chewing etc.

Prenatal causes

Children who are born with a very low birth weight, either because of prematurity or impaired growth during pregnancy due to maternal disorders can also be underweight. Such children, in the absence of any other serious illness, catch up in their weight and height usually by 2 years of age. Some children may also have any underlying genetic condition affecting their growth potential.


All children who are underweight may not have any underlying disease process. As growth is also determined by our genetic potential, children can be healthy but constitutionally thin, more so when there is a family history of the same. However, it is best to visit your pediatrician when you have concerns, so that they can make the right diagnosis for your child


Any child who appears to be malnourished, has poor growth compared with other children of their age, has a chronic disease, often falls sick or born prematurely or with low birth weight should be evaluated by a specialist. It is also important to be on regular follow up with your doctor to track your child’s growth. Treating underweight children depends on the cause. If related to nutrition, your pediatrician will advise regarding how to improve your child’s diet and address issues relating to feeding. If there is suspicion of any underlying disease, your doctor may advise some tests and suggest appropriate treatment.