Babies whose weight falls below normal levels face a number of obstacles from the moment of birth and for years afterward. Both babies born prematurely and babies who are small for their gestational age can weigh less than 5.5 pounds, the definition for low birth weight. Babies born at a normal weight who fail to gain weight normally also face potentially serious health risks. The effects of low weight in infants depends on the cause.
Low Birth Weight from Prematurity
Premature babies start out life not quite ready for life outside the womb. A preemie might have immature lungs that often can't handle the stress of breathing. A preemie's immature digestive tract often can't break down and absorb food. Because a preemie's immune system hasn't fully developed, he's prone to infections. A preemie often forgets to breathe and needs medications to stimulate his respiratory drive, even if he can breathe without supplemental oxygen. Many of the treatments given to enable preemie survival can also cause long-term damage; breathing treatments can cause long-term lung damage, for example. A very early preemie might have bleeding in the brain that can cause stroke. Some preemies suffer damage to the blood vessels in the eye, causing vision loss.
Small for Gestational Age Risks
A baby who is small for gestational age often didn't get the nutrition he needed in utero. Being small-for-gestational age -- sometimes called intrauterine growth restriction -- has different effects on an infant than prematurity, even though two babies might be about the same size. A baby born with SGA because of placental problems might weigh less than normal but still have a normal head size and length. Given proper nutrition after birth, he can catch up and have no long-term consequences from being initially underweight. Congenital or chromosomal disorders can also cause SAG; in this case, he might be proportionally small, with a small head and decreased brain growth that affects his mental development.
Poor Weight Gain After Birth
Your doctor may classify a baby with consistently poor weight gain in the first 12 months of life as having "failure to thrive." A baby with failure to thrive might be weak, susceptible to infection and might not meet developmental milestones such as sitting, crawling or walking at normal ages. Failure to thrive can also affect your baby's speech and social interactions; a baby with this condition might lose interest in his surroundings and have poor eye contact. Because a baby's brain grows more in his first year than it ever will again, in severe cases, failure to thrive can cause permanent brain damage, the Kids Health website cautions.
Not all low weight babies have short-term or long-term health problems. Some babies are genetically built thinner than others; breast-fed babies might also weigh less than bottle-fed babies after the first 6 months of life, according to pediatrician and author Dr. William Sears. Ask your baby's doctor if you're concerned about her weight; he will likely be concerned if your baby's weight or weight-to-height ratio falls more than two standard deviations below the normal for her age and gender on a standard height-weight chart.