Infant low birth weight (LBW) happens when babies weigh less than 5 pounds and 8 ounces at birth. LBW often occurs in babies who are born prematurely, before 37 weeks of gestation. It’s also common in multiple birth situations, such as cases of twins or triplets.

According to the Lucile Packard Children’s Hospital (LPCH) at Stanford, over 8 percent of all babies born in the U.S. each year have LBW. The number is increasing, potentially due to the rise of multiple births. Developing countries have much higher incidences of infants with LBW than the U.S.

If your baby is born with LBW, they will appear smaller than newborns. They’ll likely be thin, have minimal body fat, and have a disproportionately large head.

Most cases of LBW are caused by premature birth. Since babies grow a lot in the later stages of pregnancy, many babies born before the 37th week of gestation are small and underweight.

Other conditions can also cause your baby to be born with LBW. For example, LBW may be caused by:

  • problems with the birth mother’s placenta
  • complications with the birth mother’s pregnancy
  • intrauterine growth restriction (IUGR)
  • birth defects

Poor maternal nutrition, maternal drug or alcohol abuse, or incomplete prenatal care can also raise your baby’s risk of LBW.

If your baby is born with LBW, they’ll have a higher risk of developmental difficulties, health complications, and premature death than babies born at a normal weight. They may be weaker than babies with normal birth weight. They may also have trouble eating, gaining weight, staying warm, and warding off illness and infections.

Some common health complications associated with LBW include:

  • underdeveloped lungs or other organs
  • respiratory problems
  • digestive problems
  • neurological problems
  • eye or ear problems
  • sudden infant death syndrome (SIDS)

The lower your baby’s birth weight, the greater their risk of complications.

Your baby will be weighed immediately after birth. If your baby weighs less than 5 pounds and 8 ounces, they’ll be diagnosed with LBW. If they weigh less than 3pounds and 5 ounces, they’ll be diagnosed with very low birth weight (VLBW).

During proper prenatal care, your doctor should monitor your baby’s approximate size and weight while in utero. This can help your doctor identify a potential case of LBW early on, even before your baby is born.

Your baby’s recommended treatment plan for LBW will depend on their specific situation. If they’re born with LBW, they may need to stay in the hospital until they gain enough weight to be discharged.

If your baby has other complications, such as underdeveloped lungs or intestinal problems, they’ll likely need to stay in the hospital until the complications have been addressed through medical care. Your baby may receive care in the neonatal intensive care unit (NICU), where healthcare professionals can use special temperature-controlled beds and feeding techniques to care for them.

According to the World Health Organization, babies with LBW should be fed with their mother’s breast milk whenever possible. Breastmilk (and breastfeeding if possible) can help promote growth and weight gain. If their birth mother’s breast milk isn’t available, human donor milk may be used. Formula should be considered as a last resort for nutrition.

Infants born with LBW but no other complications often develop normally. In some cases, your baby may experience developmental delays, minor mental disabilities, or health problems that persist throughout their life, with varying degrees of severity.

If your baby is born with both LBW and other complications, their outlook will depend on the specific health challenges they face. Advances in medicine have increased the survival rates and improved the long-term outlook for many babies with LBW and related complications. Ask your baby’s doctor for more information about their specific condition, treatment options, and long-term outlook.