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Babies born weighing less than 5 pounds, 8 ounces (2,500 grams) are considered low birthweight. Low-birthweight babies are at increased risk for serious health problems as newborns, lasting disabilities and even death.

About 1 in every 12 babies in the United States is born with low birthweight (1). Advances in newborn medical care have greatly reduced the number of deaths associated with low birthweight. However, a small percentage of survivors develop intellectual disabilities, learning problems, cerebral palsy and vision and hearing loss.

Why are babies born with low birthweight?
There are two main reasons why a baby may be born with low birthweight:

• Premature birth: Babies born before 37 completed weeks of pregnancy are called premature. About 67 percent of low-birthweight babies are premature (1). The earlier a baby is born, the less she is likely to weigh. Very low-birthweight babies (those who weigh less than 3 pounds, 5 ounces or 1,500 grams) have the highest risk for health problems. Some premature babies born near term do not have low birthweight, and they may have only mild or no health problems as newborns.
• Fetal growth restriction: These babies are called growth-restricted, small-for-gestational age or small-for-date. These babies may be full term, but they are underweight. Some of these babies are healthy, even though they are small. They may be small simply because their parents are smaller than average. Others have low birthweight because something slowed or halted their growth in the uterus.

Some babies are both premature and growth-restricted. These babies are at high risk for health problems.

What causes low birthweight?
Preterm labor, labor that happens before 37 completed weeks of pregnancy, frequently results in the birth of a premature, low-birthweight baby. The causes of preterm labor are not thoroughly understood. However, we do know that women with these risk factors are at increased risk for delivering prematurely:

• Had a premature baby in a previous pregnancy
• Are pregnant with twins, triplets or more
• Have certain abnormalities of the uterus or cervix

Other factors that may contribute to premature birth and/or fetal growth restriction include:

• Birth defects: Babies with certain birth defects are more likely to be growth restricted because genetic conditions and structural abnormalities may limit normal development (2, 3). Babies with birth defects also are more likely to be born prematurely (4).
• Chronic health problems in the mother: Maternal high blood pressure, diabetes, and heart, lung and kidney problems sometimes can reduce birthweight (2, 3).
• Smoking: Pregnant women who smoke cigarettes are nearly twice as likely to have a low-birthweight baby as women who do not smoke (5). Smoking slows fetal growth and increases the risk of premature delivery (5).
• Alcohol and illicit drugs: Alcohol and illicit drugs can limit fetal growth and can cause birth defects (2, 3). Some drugs, such as cocaine, also may increase the risk of premature delivery.
• Infections in the mother: Certain infections, especially those involving the uterus, may increase the risk of preterm delivery (6).
• Infections in the fetus: Certain viral and parasitic infections, including cytomegalovirus, rubella, chickenpox and toxoplasmosis, can slow fetal growth and cause birth defects (2, 3).
• Placental problems: Placental problems can reduce flow of blood and nutrients to the fetus, limiting growth. In some cases, a baby may need to be delivered early to prevent serious complications in mother and baby.
• Inadequate maternal weight gain: Women who don’t gain enough weight during pregnancy increase their risk of having a low-birthweight baby (2, 6). Women of normal weight should usually gain 25 to 35 pounds during pregnancy.
• Socioeconomic factors: Low income and lack of education are associated with increased risk of having a low-birthweight baby, although the underlying reasons for this are not well understood. Black women and women under 17 and over 35 years of age also are at increased risk (2).

What can a woman do to reduce her risk of having a low-birthweight baby?
A woman can do the following before and during pregnancy to reduce her risk of having a low-birthweight baby:

• See her health care provider for a preconception checkup. Her provider can help make sure the woman is as healthy as possible before she conceives. At this visit, the provider can screen her for certain health problems (diabetes, high blood pressure, thyroid disease) and various infections, make sure her vaccinations are up-to-date, and discuss her health habits and nutrition. The provider can make sure any medications the woman takes are the safest possible choices during pregnancy.
• Work with her health care provider to control chronic health conditions, such as high blood pressure and diabetes. Good control of these conditions, starting before pregnancy, reduces the risk of low birthweight and other pregnancy complications.
• Take a multivitamin containing 400 micrograms of folic acid daily, starting before pregnancy. When taken before and early in pregnancy, folic acid helps prevent certain serious birth defects of the brain and spine. When taken throughout pregnancy, folic acid may help reduce the risk of having a premature and low-birthweight baby (7).
• Stop smoking before she becomes pregnant and remain smoke-free throughout pregnancy. A woman’s health care provider can refer her to a smoking-cessation program or suggest other ways to help her quit.
• Get early and regular prenatal care. This allows the health care provider to identify and treat problems early, which may reduce the risk of having a low-birthweight baby.
• Call her health care provider immediately if she suspects she may be having preterm labor. Her provider may want to examine her and do some tests to see if she really is in labor.
• If she is in labor, the provider may give her a medication (called a tocolytic) to try to delay or stop delivery. These drugs are most effective when given early in labor. Tocolytics often postpone delivery for only a day or two, but even such a short delay can make a difference in the baby’s health.
• If she has already had a premature baby in a prior pregnancy, ask her provider if she could benefit from treatment with the hormone progesterone. The injected form of progesterone is called 17 alpha-hydroxyprogesterone caproate (17P). Studies show that this treatment appears to reduce the risk of having another premature baby by about one-third (8).

One recent study also found that treatment with vaginal progesterone suppositories reduced the rate of premature birth in women who had a short cervix (diagnosed with an ultrasound examination), most of whom had no prior history of premature birth (9).

How is fetal growth restriction treated?
About 10 percent of fetuses are growth-restricted (3, 6). A health care provider 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 ultrasounds that monitor how quickly the fetus is growing. In some cases, fetal growth can be improved by treating any condition in the mother (such as high blood pressure) that may be a contributing factor.

The provider closely monitors the well-being of a growth-restricted fetus using ultrasound and fetal heart rate monitoring. If these tests show that the baby is having problems, the baby may need to be delivered early.

What medical problems are common 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 a newborn intensive care unit (NICU). Serious medical problems are most common in babies born at very low birthweight:

• Respiratory distress syndrome (RDS): This breathing problem is common in babies born before the 34th week of pregnancy. Babies with RDS lack a protein called surfactant that keeps small air sacs in the lungs from collapsing. Treatment with surfactant helps affected babies breathe more easily. Babies with RDS may need additional oxygen and mechanical breathing assistance to keep their lungs expanded. The sickest babies may temporarily need the help of mechanical ventilation to breathe for them while their lungs mature.
• Bleeding in the brain (called intraventricular hemorrhage or IVH): Bleeding in the brain occurs in some very low-birthweight premature babies, usually in the first three days of life. Brain bleeds usually are diagnosed with an ultrasound. Most brain bleeds are mild and resolve themselves with no or few lasting problems. More severe bleeds can cause 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 sometimes can reduce fluid buildup.
• Patent ductus arteriosus (PDA): PDA is a heart problem that is common in premature babies. Before birth, a large artery called the ductus arteriosus lets the blood bypass the baby’s nonfunctioning lungs. The ductus normally closes after birth so that blood can travel to the lungs and pick up oxygen. When the ductus does not close properly, it can lead to heart failure. PDA can be diagnosed with a special form of ultrasound (echocardiography) or other imaging tests. Babies with PDA are treated with a drug that helps close the ductus, although surgery may be necessary if the drug doesn’t work.
• Necrotizing enterocolitis (NEC): This potentially dangerous intestinal problem usually develops two to three weeks after birth. It can lead to feeding difficulties, abdominal swelling and other complications. Babies with NEC are treated with antibiotics and fed intravenously (through a vein) while the intestine heals. In some cases, surgery is necessary to remove damaged sections of intestine.
• Retinopathy of prematurity (ROP): ROP is an abnormal growth of blood vessels in the eye that can lead to vision loss. It occurs mainly in babies born before 32 weeks of pregnancy. Most cases heal themselves with little or no vision loss. In severe cases, the ophthalmologist (eye doctor) may treat the abnormal vessels with a laser or with cryotherapy (freezing) to preserve vision.

Can medical problems in premature, low-birthweight newborns be prevented?
When a provider suspects that a woman may deliver before 34 weeks of pregnancy, he may suggest treating the mother with a medicine called corticosteroids. Corticosteroids speed maturation of the fetal lungs and significantly reduce the risk of RDS, IVH, NEC and infant death. These drugs are given by injection (a shot) and are most effective when administered at least 24 hours before delivery. Treatment with tocolytic drugs to delay labor can give corticosteroids time to work. The provider also can arrange for delivery in a hospital with a NICU that can give specialized care to a premature, low-birthweight infant.

Does low birthweight contribute to adult health problems?
Some studies suggest that individuals who were born with low birthweight may be at increased risk for certain chronic conditions in adulthood. These conditions include high blood pressure, type 2 (adult-onset) diabetes and heart disease. When these conditions occur together, they are called metabolic syndrome. One study found that men who weighed less than 6 1/2 pounds at birth were 10 times more likely to have metabolic syndrome than the men who weighed more than 9 1/2 pounds at birth (10, 11).

It is not yet known how low birthweight contributes to these adult conditions. However, it is possible that growth restriction before birth may cause lasting changes in certain insulin-sensitive organs like the liver, skeletal muscles and pancreas. Before birth, these changes may help the malnourished fetus use all available nutrients. However, after birth these changes may contribute to health problems.

Is the March of Dimes supporting research on low birthweight?
The March of Dimes has long supported research on low birthweight and the related issue of prematurity. From 2004 to 2008, the March of Dimes awarded approximately $11.5 million to grantees as part of the Prematurity Research Initiative, which aims to learn more about the causes of prematurity. These grantees are exploring the role of genes, uterine muscle activity, infections, immune system changes and lung activity in triggering preterm labor, which may lead to better ways to prevent or treat it.

Other grantees are seeking to improve the treatment for premature, low-birthweight babies. For example, some are attempting to develop treatment that can help prevent brain damage and cerebral palsy in premature infants. Others are seeking to develop improved treatments for PDA, NEC and ROP. One grantee is studying the role of a family of proteins on development and functioning of the placenta, in order to develop treatments for placental problems that contribute to fetal growth restriction and premature birth.

The March of Dimes also promotes the health benefits of smoking prevention and cessation by providing educational materials for consumers, promoting evidence-based smoking-cessation methods, and supporting projects that increase smoking-cessation services available to pregnant women who smoke.

References

1. Martin, J.A., et al. (2007). Births: Final Data for 2005. National Vital Statistics Reports, 56(6).
2. American College of Obstetricians and Gynecologists (ACOG). Intrauterine Growth Restriction. ACOG Practice Bulletin, number 12, January 2000.
3. Berghella, V. (2007). Prevention of Recurrent Fetal Growth Restriction. Obstetrics and Gynecology, 110(4), 904-912.
4. Honein, M.A., et al. (2008). The Association between Major Birth Defects and Preterm Birth. Maternal and Child Health Journal, 12:(4).
5. U.S. Department of Health and Human Services. The Health Consequences of Smoking: A Report of the Surgeon General, 2004. Centers for Disease Control and Prevention, Office on Smoking and Health, Atlanta, GA, May 2004.
6. Goldenberg, R.L., Culhane, J.F. (2007). Low Birth Weight in the United States. American Journal of Clinical Nutrition, (suppl), 584S-590S.
7. Siega-Riz, A., et al. (2004). Second Trimester Folate Status and Preterm Birth. American Journal of Obstetrics and Gynecology, 191, 1851-1857.
8. Meis, P.J., et al. (2003). Prevention of Recurrent Preterm Delivery by 17 Alpha-Hydroxyprogesterone Caproate. New England Journal of Medicine, 348(24), 2379-2385.
9. Fonseca, E.B., et al. (2007). Progesterone and the Risk of Preterm Birth among Women with a Short Cervix. New England Journal of Medicine, volume 375(5), 462-469.
10. Valsmakis, G., et al. (2006). Causes of Intrauterine Growth Restriction and the Postnatal Development of the Metabolic Syndrome. Annals of the New York Academy of Sciences, 1092, 138-147.
11. Barker, D.J.P. (1993).Type 2 (Non-Insulin-Dependent) Diabetes Mellitus, Hypertension and Lyperlipidaemia (Syndrome X): Relation to Reduced Fetal Growth. Diabetologia, 36(10), 62-67.

May 2008