What is low birth weight?

Low birth weight is a term used to describe babies who are born weighing less than 5 pounds, 8 ounces (2,500 g). An average newborn usually weighs about 8 pounds. A low-birth-weight baby may be healthy even though they are small. But a low-birth-weight baby can also have many serious health problems.

What causes low birth weight?

Low birth weight is most often caused by being born too early (premature birth). That means before 37 weeks of pregnancy. A premature baby has less time in the pregnant person's womb (uterus) to grow and gain weight. Much of a baby's weight is gained during the last weeks of pregnancy.

Another cause of low birth weight is a condition called intrauterine growth restriction (IUGR). This occurs when a baby does not grow well during pregnancy. It may be because of problems with the placenta, the pregnant person's health, or the baby's health. Babies can have IUGR and be:

• Full term. That means born from 37 to 41 weeks of pregnancy. These babies may be physically mature but small.

• Premature. These babies are both very small and physically immature.

Which babies are at risk for low birth weight?

In addition to preterm birth and IUGR, other things can raise the risk of having a baby with low birth weight. They include:

• Infection during pregnancy

• Not gaining enough weight during pregnancy

• Previous pregnancy with a low-birth-weight baby

• Smoking

• Alcohol or drug use

• Age less than 17 years or more than 35 years

Pregnant people with an African-American background are more likely to have a baby with low birth weight. Researchers are working to learn why.

What are the symptoms of low birth weight?

In addition to weighing less than 5 pounds, 8 ounces, babies with low birth weight look much smaller than babies of normal birth weight. A low-birth-weight baby's head may look bigger than the rest of their body. They often look thin with little body fat.

How is low birth weight diagnosed?

One of the main reasons for regular prenatal exams is to make sure your baby is growing well. During pregnancy, the size of your baby is estimated in different ways. Your steady weight gain is one way of checking on the baby's growth. Another way is fundal height:

• To check fundal height, your healthcare provider measures from the top of your pubic bone to the top of your uterus (fundus).

• Fundal height is measured in centimeters (cm). It is about the same as the number of weeks of pregnancy after the 20th week. For example, at 24 weeks gestation, your fundal height should be close to 24 cm.

• If the fundal height is less than expected, it may mean the baby is not growing well.

Your healthcare provider may also use fetal ultrasound to check your baby's growth and development. Ultrasound uses sound waves to create a picture of your baby. It is more accurate than checking fundal height. Measurements can be taken of your baby's head, belly (abdomen), and upper leg bone (femur). These measurements are used to estimate their weight.

Babies are weighed within the first few hours after birth. The weight is compared against the number of weeks of pregnancy (gestational age). If your baby weighs less than 2,500 g (5 pounds, 8 ounces), they have a low birth weight. Babies weighing less than 1,500 g (3 pounds, 5 ounces) at birth are considered very low birth weight. Babies who weigh less than 1,000 g (2 pounds, 3 ounces) are extremely low birth weight.

How is low birth weight treated?

Treatment will depend on your child’s symptoms, age, and general health. It will also depend on how severe the condition is.

Treatment for low birth weight often includes:

• Care in the neonatal intensive care unit (NICU)

• Temperature-controlled bed

• Special feedings. Sometimes these are given through a tube into the stomach if a baby cannot suck. Or they are given through an IV (intravenous) line.

How well a baby with low birth weight does depends largely on how much the baby weighs at birth. Babies who weigh less than 1 pound, 1.5 ounces (500 g) have the most problems and are much less likely to survive.

Low-birth-weight babies typically catch up in physical growth if they have no other complications. Babies may need to have special follow-up healthcare programs.

What are possible complications of low birth weight?

Low-birth-weight babies often have problems. The baby's tiny body is not as strong as a baby of normal birth weight. They may have a harder time eating, gaining weight, and fighting infection. Low-birth-weight babies often have a hard time staying warm because they don't have much fat on their bodies.

Babies that are born premature often have complications. It is sometimes hard to tell if the problems are because they were born early or because they are so small. In general, the lower the birth weight, the greater the risk for complications. Some of the common problems of low-birth-weight babies include:

• Low oxygen levels at birth

• Trouble staying warm

• Trouble feeding and gaining weight

• Infection

• Breathing problems and immature lungs (infant respiratory distress syndrome)

• Nervous system problems, such as bleeding inside the brain (intraventricular hemorrhage)

• Digestive problems, such as serious inflammation of the intestines (necrotizing enterocolitis)

• SIDS (sudden infant death syndrome)

• Yellow color to the skin or eyes (jaundice)

• Persistent ductus arteriosus. This happens when an open blood vessel needed for fetal circulation does not close correctly at birth. This causes extra blood to flow through the lungs.

• Retinopathy of prematurity. This is a disease of the eye where the retina does not develop correctly.

Babies with very low birth weight are at risk for long-term complications and disability. Long-term complications may include:

• Cerebral palsy

• Blindness

• Deafness

• Developmental delay 

Talk with your baby's healthcare provider for information about your baby's risks for complications.

Can low birth weight be prevented?

More babies are surviving even though they are born early and are very small. This is because of advances in the care of sick and premature babies. But preventing preterm births is one of the best ways to prevent babies being born with low birth weight.

Regular prenatal care is the best way to prevent preterm births and low-birth-weight babies. At prenatal visits, the healthcare provider will check the health of you and your baby. It is important to:

• Follow a healthy diet during pregnancy. This will help you to gain enough weight to help your baby grow and help you stay healthy.

• Not drink alcohol, smoke, or use drugs. All of these can cause low birth weight and other problems for your baby.

Key points about low birth weight

• Low birth weight is a term used to describe babies who are born weighing less than 5 pounds, 8 ounces (2,500 g).

• Babies weighing less than 3 pounds, 5 ounces (1,500 g) at birth are considered very low birth weight.

• Low birth weight is most often caused by premature birth.

• Nearly all low-birth-weight babies need special care in the neonatal intensive care unit (NICU) until they gain weight and are well enough to go home.

• Regular prenatal care is the best way to prevent preterm births and low-birth-weight babies.

Next steps

Tips to help you get the most from a visit to your child’s healthcare provider:

• Know the reason for the visit and what you want to happen.

• Before your visit, write down questions you want answered.

• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you for your child.

• Know why a new medicine or treatment is prescribed and how it will help your child. Also know what the side effects are.

• Ask if your child’s condition can be treated in other ways.

• Know why a test or procedure is recommended and what the results could mean.

• Know what to expect if your child does not take the medicine or have the test or procedure.

• If your child has a follow-up appointment, write down the date, time, and purpose for that visit.

• Know how you can contact your child’s healthcare provider after office hours. This is important if your child becomes ill and you have questions or need advice.