Children go through phases of weight gain as they grow, and your child's doctor probably won't be too concerned by a single instance of lack of weight gain. However, if your child doesn't seem to be gaining weight over time or slips significantly behind on the weight growth charts, she might be experiencing a "failure to thrive." This typically occurs in infants and toddlers, according to KidsHealth, but there are also reasons older children and teenagers don't gain weight.
Children who aren't getting the nourishment they need will not gain weight. This might occur when a family is having economic struggles, when a toddler is a supremely picky eater or when parents intentionally limit food intake to prevent the child from "getting fat." When an undernourished child starts eating more food, she'll start gaining weight again. If obtaining food is difficult, consider applying for the Supplemental Nutrition Assistance Program or look for food banks in your area. You can also build meals based on cheaper -- but still nutritious -- food items. Picky eaters may be attracted to nutritional shakes that offer more calories and nutrition. You can also beef up their intake by using full-fat versions of dairy products.
Some children have allergies or intolerances that make it difficult for the body to absorb nutrition. Dairy and wheat intolerances are often prime suspects. If you're not sure if your child has this problem, you can ask for testing or try eliminating these products from his diet for a few weeks to see if the situation improves. Once you have an official diagnosis, you may find that the way it limits food choices can also contribute to lack of weight gain, requiring extra effort in finding nutritious foods your child can eat.
Certain illnesses can also prevent weight gain. For example, gastroesophageal reflux disease (GERD) brings food back up through the esophagus, which leads to pain and a refusal to eat. Chronic diarrhea, cystic fibrosis or celiac disease may mean that your child is not absorbing nutrients properly. Note that a child who's not gaining weight, but still growing taller is experiencing a loss of body mass. Diseases that can cause weight loss or a failure to gain weight include Crohn's disease, cancer, diabetes, hypothyroidism or hyperthyroidism, along with others, according to the Mayo Clinic. These diseases all have other symptoms that go along with weight loss, so consider other problems your child may be having and discuss them with her doctor before jumping to conclusions.
According to HealthyChildren.org, the website of the American Academy of Pediatrics, 90 percent of those who suffer from disordered eating are girls between the ages of 12 and 25. Disordered eating is not limited to teen girls, though. Younger girls may start to feel uncomfortable in their bodies, especially if they are going through puberty early, and even boys can be unhappy with the way they look. Look for signs that your child is feeling down about his body, exercising a lot and strictly watching what he eats.