An infant who weighs less than 5 pounds, 8 ounces at birth is classified as a low-birth-weight baby, explains MedlinePlus, a website published by the National Institutes of Health. Weighing less than 3 pounds, 5 ounces is considered to be a very low birth weight. Being born prematurely -- prior to the 37th week of gestation -- is one of two primary reasons a newborn will weigh less than the average 6- to 9-pound full-term baby. Low-birth-weight babies and "preemies" have more similarities than they do differences.
Premature birth is the leading cause of low birth weight. In fact, seven out of 10 low weight newborns are premature, according to the March of Dimes. Intrauterine growth restriction is the second leading cause of low birth weight, reports Boston Children's Hospital. IUGR can occur due to abnormalities with placenta, birth defects or the mother's health. Newborns with IUGR can be full-term or premature. Preemies with IUGR are typically tiny and physically underdeveloped while full-term infants with the condition tend to be weak even though they're fully developed physically. Fetal growth restriction refers to a baby who fails to gain enough weight before birth.
Mother's Lifestyle and Health
Women who smoke while pregnant are far more likely to give birth to a smaller than average baby than women who don't smoke. Smoking retards a baby's growth and increases the risk of going into labor prematurely. Drinking alcohol, using cocaine and other street drugs slows a baby's growth and increases the chances of premature birth. Mind-altering substances may also cause birth defects like abnormally formed faces or heads, missing or undersized arms and legs and cleft palates, notes WebMD. A baby is more inclined to suffer from fetal growth restriction and/or be born prematurely when his mother suffers from diabetes, high blood pressure, heart, lung and kidney conditions.
Since many low-birth-weight babies are also premature, it can be hard to distinguish whether potential medical problems are attributable to prematurity or to the fact that a full-term baby is smaller than average, explains Boston Children's Hospital. Low-birth weight newborns -- both full-term and preemies -- aren't as strong as normal-weight babies, which can make it tough to fend off infections. Breathing problems include respiratory distress syndrome, a disease that preemies may succumb to due to their immature lungs. Full-term babies rarely develop RDS, notes the American Thoracic Society. Bleeding inside the brain and gastrointestinal issues are other potential complications typically associated with premature births.
Ongoing prenatal care can help ensure that your baby is born full-term at a healthy weight. Eating a nutritious diet and gaining enough weight during pregnancy is critical to the health of your baby. The American Pregnancy Association says a healthy pregnancy weight gain for a normal size woman is 25 to 35 pounds. Very thin or heavy women may be advised to gain more or less weight.
Nearly all low-birth-weight infants require specialized care in the neonatal intensive care unit until they gain weight and are healthy enough to go home. Babies who weigh between 3 pounds, 5 ounces and 5 pounds, 8 ounces have a 95-percent survival rate. A preemie or low-birth-weight baby isn't destined to be a small child, teen or adult. Most infants who were born prematurely or weighed less than average at birth grow up to be about the same size as their parents.