During infancy and childhood, children gain weight and grow more rapidly than at any other time in life. However, some children do not gain weight at a normal rate, either because of expected variations related to genes, being born prematurely, or because of under nutrition, which may occur for a variety of reasons. Under nutrition is sometimes called a growth deficit or failure to thrive. It is important to recognize and treat children who are not gaining weight normally because it may be a sign of under nutrition or an underlying medical problem that requires treatment. Under nutrition can have complications, such as a weakened immune system, slower than expected linear growth, shorter than expected height, or difficulties with learning. These complications are more common in children who are undernourished for a long period of time.