Birth weight is the first weight of your baby, taken just after he or she is born. A low birth weight is less than 5.5 pounds. A high birth weight is more than 8.8 pounds.
A low birth weight baby can be born too small, too early (premature), or both. This can happen for many different reasons. They include health problems in the mother, genetic factors, problems with the placenta and substance abuse by the mother.
Some low birth weight babies may be more at risk for certain health problems. Some may become sick in the first days of life or develop infections. Others may suffer from longer-term problems such as delayed motor and social development or learning disabilities.
High birth weight babies are often big because the parents are big, or the mother has diabetes during pregnancy. These babies may be at a higher risk of birth injuries and problems with blood sugar.
Centers for Disease Control and Prevention