What is very low birth weight?

Very low birth weight (VLBW) is a term used to describe babies who are born weighing less than 3 pounds, 4 ounces (1.5 kilograms). It is very rare that babies are born this tiny. Only about 1% of babies born in the U.S. are very low birth weight.

What causes very low birth weight?

The main cause of a baby having VLBW is being born too early. This is called preterm or premature birth. Premature means a baby is born before 37 weeks of pregnancy. Very-low-birth-weight babies are often born before 30 weeks of pregnancy. A premature baby has less time in the pregnant person’s uterus to grow and gain weight. Much of a baby's weight is gained during the later part of pregnancy.

Another cause of very low birth weight is when a baby does not grow well during pregnancy. This is called fetal growth restriction (FGR). In the past, this was called intrauterine growth restriction. It may happen because of problems with the placenta, the pregnant person's health, or birth defects. Most very low birth weight babies who have FGR are also born early. They are usually very small and physically immature.

Who is at risk for very low birth weight?

A baby is more likely to be very low birth weight if they are premature or have had fetal growth restriction. Other things that happen to or with the pregnant person can increase the risk for VLBW, growth restriction, and prematurity. They include:

• Having an infection during pregnancy

• Not gaining enough weight during pregnancy

• Having a previous pregnancy with a low-birth-weight baby

• Smoking

• Using alcohol or illegal drugs

• Being younger than age 17 or older than age 35

• Being African American

What are the symptoms of very low birth weight?

Babies with very low birth weight look much smaller than babies of normal birth weight. A very-low-birth-weight baby's head may look big compared with the rest of their body. A baby with VLBW often looks very thin with little body fat. Blood vessels can be easily seen through the skin.

How is very low birth weight diagnosed?

One of the main reasons for regular prenatal exams is to make sure your unborn baby is growing well. During pregnancy, the size of your baby is estimated in different ways. Your steady weight gain is 1 way of checking on the baby’s growth.

Another way is to measure the top of your uterus from the pubic bone (fundal height). The number of centimeters measured is usually the same or close to the number of weeks of pregnancy. If the fundal height measurement is low for the number of weeks you are pregnant, it may mean that your baby is not growing well.

Other ways to check the baby’s growth are:

• Using ultrasound to estimate the baby’s growth and development. Your healthcare provider can use measurements of your baby's head, belly, and upper leg bone to estimate their weight.

• Comparing your baby’s estimated birth weight with their gestational age. The provider may use a formula to figure out your baby’s body mass.

A birth weight of less than 5 pounds, 8 ounces (2.5 kilograms) is called low birth weight. Babies weighing less than 3 pounds, 4 ounces (1.5 kilograms) at birth are considered very low birth weight.

How is very 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. Babies with VLBW need:

• Care in the neonatal intensive care unit (NICU)

• Temperature-controlled beds

• Special feedings, most often with a tube into the stomach as well as IV fluid

• Other treatments for possible complications

The outcome for a baby with VLBW depends mainly on how much the baby weighs and how many weeks of gestation the baby is at birth. The smallest and earliest babies have the most problems. They are less likely to survive.

Babies with VLBW may have a harder time catching up in physical growth because they often have other problems. Many very-low-birth-weight babies are referred to special follow-up healthcare programs.

What are possible complications of very low birth weight?

Babies with a very low birth weight have a greater risk of developing problems. Their tiny bodies are not as strong as babies of normal weight. They may have a harder time eating, gaining weight, and fighting infection. They have very little body fat. So they often have trouble staying warm in normal temperatures.

Most babies with a very low birth weight are also premature. This can make it hard to separate the problems caused by the prematurity from the problems of just being so tiny. In general, the lower the baby's birth weight, the greater the risks for complications. Here are some possible problems for babies with VLBW:

• Low blood sugar levels at birth

• Retinopathy of prematurity, an eye disorder that can cause blindness

• Low oxygen levels at birth

• Trouble staying warm

• Trouble feeding and gaining weight

• Infection

• Breathing problems because of immature lungs (respiratory distress syndrome)

• Nervous system problems, such as bleeding inside the brain or damage to the brain’s white matter

• Serious digestive problems, such as necrotizing enterocolitis

Very-low-birth-weight babies need special care in the NICU until they can gain weight and are well enough to go home.

Risks for long-term problems and disability are increased for babies with VLBW. Long-term complications may include:

• Cerebral palsy or difficulty with movement

• Blindness or difficulty with vision (for example, may need glasses)

• Deafness or difficulty with hearing (for example, may need hearing aids)

• Developmental delay including behavioral, emotional, and/or learning problems

Talk with your baby's healthcare provider about your baby’s risks for complications. Ask about what can be done to take care of these as early as possible. Also ask whether your child will need checkups or screening as they grow up.

Can very low birth weight be prevented?

There have been advances in the care of sick and premature babies. More babies today survive even though they are born early and very small. But preventing preterm births is 1 of the best ways to prevent very low birth weight.

Regular prenatal care is the best way to prevent preterm births and very-low-birth-weight babies. At prenatal visits, your health and the health of your unborn baby are checked. It is important to:

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

• Stay away from alcohol, cigarettes, or illegal drugs.  All of these can cause low birth weight and other problems for your baby. 

Key points about very low birth weight

• Very low birth weight is a term used to describe babies who are born weighing less than 3 pounds, 4 ounces (1.5 kilograms).

• The main cause of very low birth weight is being born too early, called premature birth.

• Treatment for very low birth weight babies includes care in the NICU, temperature-controlled beds, and special feedings.

• In general, the lower the baby's birth weight, the greater the risks for complications.

• Prenatal care is a key factor in preventing preterm births and very low birth weight.

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 provider after office hours. This is important if your child becomes ill and you have questions or need advice.