Low birthweight is when a baby is born weighing less than 5 pounds, 8 ounces. Some babies with low birthweight are healthy, even though they’re small. But having a low weight at birth can cause serious health problems for some babies. A baby who is very small at birth may have trouble eating, gaining weight and fighting off infections. Some may have long-term health problems, too. About 1 in 12 babies (about 8 percent) in the United States is born with low birthweight.