Low BirthweightBajo Peso al Nacer

Low Birthweight

What is low birthweight?

Low birthweight is a term used to describe babies who are born weighing less than 2,500 grams (5 pounds, 8 ounces). In contrast, the average newborn weighs about 8 pounds. Over 8 percent of all newborn babies in the United States have low birthweight. The overall rate of these very small babies in the United States is increasing. This is primarily due to the greater numbers of multiple birth babies who are more likely to be born early and weigh less. Over half of multiple birth babies have low birthweight compared with only about 6 percent of single birth babies.

Babies with low birthweight look much smaller than other babies of normal birthweight. A low birthweight baby's head may appear to be bigger than the rest of the body, and he or she often looks thin with little body fat.

What causes low birthweight?

The primary cause of low birthweight is premature birth (being born before 37 weeks gestation). Being born early means a baby has less time in the mother's uterus to grow and gain weight. Much of a baby's weight is gained during the latter part of pregnancy.

Another cause of low birthweight is intrauterine growth restriction (IUGR). This occurs when a baby does not grow well during pregnancy because of problems with the placenta, the mother's health, or the baby's condition. A baby can have IUGR and be born at full term (37 to 41 weeks). Babies with IUGR born at term may be physically mature but may be weak. Premature babies that have IUGR are both very small and physically immature.

Who is affected by low birthweight?

Any baby born prematurely is more likely to be very small. However, there are other factors that can also contribute to the risk of very low birthweight. These include:

  • Race. African-American babies are two times more likely to have low birthweight than white babies.

  • Age. Teen mothers (especially those younger than 15 years old) have a much higher risk of having a baby with low birthweight.

  • Multiple birth. Multiple birth babies are at increased risk for low birthweight because they often are premature. Over half of twins and other multiples have low birthweight.

  • Mother's health. Babies of mothers who are exposed to illicit drugs, alcohol, and cigarettes are more likely to have low birthweight. Mothers of lower socioeconomic status are also more likely to have poorer pregnancy nutrition, inadequate prenatal care, and pregnancy complications--all factors that can contribute to low birthweight.

Why is low birthweight a concern?

A baby with low birthweight may be at increased risk for complications. The baby's tiny body is not as strong and he or she may have a harder time eating, gaining weight, and fighting infection. Because they have so little body fat, low birthweight babies often have difficulty staying warm in normal temperatures.

Because many babies with low birthweight are also premature, it is can be difficult to separate the problems due to the prematurity from the problems of just being so tiny. In general, the lower the birthweight, the greater the risk for complications. The following are some of the common problems of low birthweight babies:

  • Low oxygen levels at birth

  • Inability to maintain body temperature

  • Difficulty feeding and gaining weight

  • Infection

  • Breathing problems, such as infant respiratory distress syndrome (a respiratory disease of prematurity caused by immature lungs)

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

  • Gastrointestinal problems, such as necrotizing enterocolitis (a serious disease of the intestine common in premature babies)

  • Sudden infant death syndrome (SIDS)

Nearly all low birthweight babies need specialized care in the Neonatal Intensive Care Unit (NICU) until they gain weight and are well enough to go home. Survival of infants with low birthweight depends largely on how much the baby weighs at birth, with the smallest babies (<500 grams) having the lowest survival rate. 

How is low birthweight diagnosed?

During pregnancy, a baby's birthweight can be estimated in different ways. The height of the fundus (the top of a mother's uterus) can be measured from the pubic bone. This measurement in centimeters usually corresponds with the number of weeks of pregnancy after the 20th week. If the measurement is low for the number of weeks, the baby may be smaller than expected. Ultrasound (a test using sound waves to create a picture of internal structures) is a more accurate method of estimating fetal size. Measurements can be taken of the fetus' head, abdomen, and femur and compared with a growth chart to estimate fetal weight.

Babies are weighed within the first few hours after birth. The weight is compared with the baby's gestational age and recorded in the medical record. A birthweight less than 2,500 grams (5 pounds, 8 ounces) is diagnosed as low birthweight. Babies weighing less than 1,500 grams (3 pounds, 5 ounces) at birth are considered very low birthweight.

Treatment for low birthweight

Specific management for low birthweight will be determined by your baby's doctor based on:

  • Your baby's gestational age, overall health, and medical history

  • Your baby's tolerance for specific medications, procedures, or therapies

  • Your opinion or preference

Care for low birthweight babies often includes:

  • Care in the NICU

  • Temperature controlled beds

  • Special feedings, sometimes with a tube into the stomach if a baby cannot suck, or through an intravenous (IV) line 

  • Other treatments for complications

Low birthweight babies typically "catch up" in physical growth if there are no other complications. Babies may be referred to special follow-up health care programs.

Prevention of low birthweight

Because of the tremendous advances in care of sick and premature babies, more and more babies are surviving despite being born early and being born very small. However, prevention of preterm births is one of the best ways to prevent babies born with low birthweight.

Prenatal care is a key factor in preventing preterm births and low birthweight babies. At prenatal visits, the health of both mother and fetus can be checked. Because maternal nutrition and weight gain are linked with fetal weight gain and birthweight, eating a healthy diet and gaining the proper amount of weight in pregnancy are essential. Mothers should also avoid alcohol, cigarettes, and illicit drugs, which can contribute to poor fetal growth, among other complications.