Low birthweight affects about one in every 13 babies born each year in the United States. It is a factor in 65 percent of infant deaths. Low birthweight babies may face serious health problems as newborns, and are at increased risk of long-term disabilities.
Advances in newborn medical care have greatly reduced the number of infant deaths associated with low birthweight, as well as the number of disabilities survivors of low birthweight experience. Still, a small percentage of survivors are left with problems such as mental retardation, cerebral palsy and impairments in lung function, sight and hearing.
What is low birthweight?
Low birthweight is a weight of less than 5 pounds, 8 ounces (2,500 grams) at birth. Very low birthweight is a weight of less than 3 pounds, 5 ounces (1,500 grams).
There are two categories:
- Preterm births (also called premature births) occur before the end of the 37th week of pregnancy. More than 60 percent of low-birthweight babies are preterm. The earlier a baby is born, the less developed its organs will be, the less it is likely to weigh, and the greater its risk for many problems. (Some premature babies born near term, at around 35 to 37 weeks, do not have low birthweight, and may have only mild or no health problems as newborns.)
- Small-for-date babies ("small for gestational age" or "growth-restricted") may be full-term but are underweight. Their low birthweight results, at least partly, from slowing or temporary halting of growth in the womb.
Some babies are both premature and growth-restricted. They are at high risk for many problems linked to low birthweight.
What causes low birthweight?
We know only some of the reasons babies are born too small, too soon, or both. Fetal defects that result from genetic conditions or environmental factors may limit normal development. Multiples (twins, triplets, or higher) often are low birthweight, even at term. If the placenta is not functioning properly, a fetus may not grow as well as it should.
A mother's medical problems influence birthweight, especially if she has high blood pressure, certain infections or heart, kidney or lung problems. An abnormal uterus or cervix can increase the mother's risk of having a premature, low-birthweight baby. However, the causes of preterm labor—which often results in a low-birthweight baby—are poorly understood.
A mother's actions before and during pregnancy may affect birthweight. All women planning pregnancy should:
- Have a pre-pregnancy checkup.
- Consume a multivitamin containing 400 micrograms of the B vitamin folic acid (the amount found in most multivitamins) every day before and in the early months of pregnancy.
- Stop smoking. Smokers have smaller babies than non-smokers, on average, and maternal exposure to another person’s smoking also may decrease the baby’s birthweight.
- Stop drinking alcohol and/or using illicit drugs, or prescription or over-the-counter drugs (including herbal preparations) not prescribed by a doctor aware of the pregnancy. Drug and alcohol use limits fetal growth and can cause birth defects.
Once pregnant:
- Get early, regular prenatal care.
- Eat a balanced diet with enough calories (usually about 300 calories a day more than a woman normally eats). Since a fetus is nourished by what a mother eats, it can suffer if the mother eats poorly.
- Gain enough weight. Health care providers recommend that a woman of normal weight gain 25 to 35 pounds during pregnancy.
Socioeconomic factors such as low income and lack of education also are associated with increased risk of having a low-birthweight baby, although the underlying reasons for this are not well understood. Women under 17 or over 35, unmarried mothers and women who have had a previous preterm birth, are at increased risk of having low-birthweight babies. Teenagers, in particular, may not have good health habits. Women who experience excessive stress and victims of domestic violence or other abuse also may be at increased risk of having a low-birthweight baby.
Can low birthweight be prevented?
A recent study showed that it is difficult to predict which women are going to deliver preterm. When a woman develops preterm labor, a doctor may recommend a medication (called a tocolytic) in an attempt to prevent preterm delivery. These drugs often postpone delivery for only a day or two, but even such a short delay can be helpful, as explained below.
Doctors continue to seek better ways to prevent and treat preterm labor. However, there are some things a pregnant woman or woman planning pregnancy can do to reduce her risk of having a low-birthweight baby. The most effective way to prevent low birthweight is to see a doctor before pregnancy and, once pregnant, get early and regular prenatal care. A pre-pregnancy visit is especially crucial for women with chronic disorders such as diabetes and high blood pressure. Good control of these disorders, starting before pregnancy, reduces the risk of pregnancy complications. All women can benefit from early advice on good nutrition, as well as about the importance of stopping risky behaviors, especially smoking, drinking alcohol and taking unprescribed drugs.
A 1996 study published in the American Journal of Clinical Nutrition suggested that consuming the recommended prenatal amount of folic acid throughout pregnancy may reduce the risk of having a preterm and low-birthweight baby.
When women receive adequate prenatal care, many problems can be identified early, allowing treatment that may reduce their risk of having a low-birthweight baby.
How is fetal growth restriction treated?
About 5 percent of fetuses are considered growth restricted. A doctor may suspect fetal growth restriction if the mother’s uterus is not growing at a normal rate. This can be confirmed with a series of ultrasound examinations that monitor how quickly the fetus is growing. In some cases, fetal growth can be improved by treating any condition in the mother that may be contributing. For example, a woman with a pregnancy-related form of high blood pressure called preeclampsia would generally be treated with bedrest and, sometimes, blood pressure medications. A woman who smokes would be counseled to stop.
The doctor will closely monitor the well-being of a growth-restricted fetus, using ultrasound and fetal heart rate monitoring. If these tests show that the baby is in trouble, due to lack of oxygen or other problems, early delivery may be recommended.
What problems occur in low-birthweight babies?
Low-birthweight babies are more likely than babies of normal weight to have health problems during the newborn period. Many of these babies require specialized care in intensive care nurseries to help them survive. Serious medical problems are most common in babies born at very low birthweight.
A low-birthweight, premature baby is at greater risk of developing breathing problems. According to the American Lung Association, about 25,000 babies a year#most of whom were born before the 34th week of pregnancy#suffer from respiratory distress syndrome (RDS). Babies with RDS lack a substance called surfactant that keeps small air sacs in the lungs from collapsing. Treatment with surfactant helps babies breathe more easily and, since it was widely introduced in 1990, infant deaths due to RDS have been reduced by two-thirds.
Babies with RDS may need additional oxygen and mechanical breathing assistance to keep their lungs expanded. The air may be delivered through small tubes in the baby’s nose, or through a tube that has been inserted into his windpipe. The tube helps the baby breathe, but does not breathe for him. The sickest babies may temporarily need the help of a respirator to breathe for them while their lungs mature. They also may be treated with a gas called nitric oxide, which can make breathing more effective by helping blood vessels in the lungs to relax.
Bleeding in the brain (called periventricular and/or intraventricular hemorrhage) occurs in 10 to 50 percent of very-low-birthweight infants, usually in the first four days of life. The bleeds are generally diagnosed with an ultrasound examination. Most are mild and resolve themselves with no or few lasting problems. More severe bleeds can cause the fluid-filled spaces (ventricles) in the brain to expand rapidly, creating pressure on the brain that can lead to brain damage. In such cases, surgeons may insert a tube into the brain to drain the fluid and reduce the risk of brain damage. In milder cases, drugs may reduce fluid buildup.
Premature babies may have a dangerous heart problem called patent ductus arteriosus (PDA). Before birth, a large artery called the ductus arteriosus lets the blood bypass the baby’s nonfunctioning lungs. In premature babies, the artery may not close properly, and even lead to heart failure. Babies with PDA are treated with a drug that helps close the ductus, though surgery may be necessary if the drug doesn’t work.
Some premature babies have a dangerous intestinal problem called necrotizing enterocolitis (NEC), which leads to feeding difficulties, abdominal swelling and other complications. Babies with NEC are treated with intravenous fluids and antibiotics while the bowel heals. In some cases, surgery is necessary to remove damaged sections of intestine.
Retinopathy of prematurity (ROP), caused by an abnormal growth of blood vessels in the eye that can lead to vision loss, occurs mainly in babies born before 32 weeks. Most cases heal by themselves with little or no vision loss. More severe cases are treated with a laser or with cryotherapy (freezing) to preserve vision.
Many premature, low-birthweight babies lack enough body fat to maintain a normal body temperature. Low body temperature can slow growth and contribute to breathing problems and other complications. These babies are placed in an incubator or warmer right after birth to help them maintain a normal body temperature. For more information on medical problems of premature infants, see the Preterm Birth Fact Sheet.
Can medical problems in low-birthweight premature newborns be prevented?
When a doctor suspects that a woman may deliver preterm, he may suggest treating her with drugs called corticosteroids. These drugs cross the placenta and speed maturation of the fetal lungs, reducing infant deaths by 30 percent and cutting the incidence of the two most serious complications of premature birth, RDS (by 50 percent) and bleeding in the brain (by 70 percent). These drugs are given by injection, and are most effective when administered at least 24 hours before delivery. This delay is a main reason for use of tocolytic drugs; the delay also allows for the mother to be transported to a medical center that is equipped to handle high-risk deliveries and care of sick and premature newborns.
Is the March of Dimes supporting research on preterm birth and low birthweight?
Since 1998, the March of Dimes has awarded more than $7 million in grants to investigate biological, social and environmental factors that may contribute to preterm or low-birthweight births. For example, researchers are looking at how genes, hormonal changes, infection, maternal stress, racism, and occupational factors may contribute to preterm labor, with the goal of learning how to prevent it.
Others are seeking to improve treatment for premature, low-birthweight babies. For example, one grantee is seeking to develop new treatments to prevent brain damage and cerebral palsy in premature babies who have suffered brain bleeds. Others are studying the roles of certain growth factors in development of retinopathy of prematurity, in order to develop better ways of preventing it, and looking for ways to make surfactant treatment even more effective.
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