Low birthweight is when a baby is born weighing less than 5 pounds, 8 ounces. About 1 in every 12 babies in the United States is born with low birthweight.
Some low birthweight babies are healthy, even though they’re small. But being low birthweight can cause serious health problems for some babies.
What causes a baby to be born with low birthweight?
There are two main reasons why a baby may be born with low birthweight:
- Premature birth. This is birth before 37 weeks of pregnancy. About 7 of 10 low-birthweight babies are premature. The earlier a baby is born, the lower her birthweight may be. About 1 in 9 babies in the United States is born prematurely. Talk to your health provider about things you can do to help reduce your chances of having a premature baby.
- Fetal growth restriction (also called growth-restricted, small for gestational age and small for date). This means a baby doesn't gain the weight she should before birth. Growth-restricted babies may have low birthweight simply because their parents are small. Others may have low birthweight because something slowed or stopped their growth in the womb. About 1 in 10 babies (10 percent) are growth-restricted. Your health care provider may think your baby isn't growing normally if your uterus (womb) isn't growing. He may use ultrasound and heart rate monitoring to check your baby’s growth and health. In some cases, a baby’s growth can be improved by treating health conditions in the mother, like high blood pressure.
Premature birth and fetal growth restriction may be caused by conditions that affect your baby in the womb. These include:
- Birth defects. These are health conditions that a baby has at birth. Birth defects change the shape or function of one or more parts of the body. They can cause problems in overall health, how the body develops, or in how the body works. They may limit a baby’s development in the womb, which may lead to low birthweight. Babies with birth defects are more likely than babies without birth defects to be born prematurely.
- Infections. Certain infections in the baby can slow growth in the womb and cause birth defects. These include cytomegalovirus, rubella, chickenpox and toxoplasmosis.
Are some women more likely than others to have a low-birthweight baby?
Yes. Things that make you more likely than others to have a low-birthweight baby are called risk factors. Having a risk factor doesn’t mean for sure that your baby will have low birthweight. But it may increase your chances. Because many low-birthweight babies are born prematurely, many risk factors for having a low-birthweight baby are the same for preterm labor and premature birth.
Talk to your health care provider about what you can do to help reduce your risk for having a low-birthweight baby.
Medical risk factors for having a low-birthweight baby
- Preterm labor. This is labor that starts too soon, before 37 weeks of pregnancy.
- Chronic health conditions. These are health conditions that last for a long time or that happen again and again over a long period of time. They need treatment from a health care provider. Chronic health conditions that may lead to low birthweight include high blood pressure, diabetes and heart, lung and kidney problems.
- Infections. Certain infections, especially infections in the uterus (womb), may increase your chances of having a premature baby.
- Problems with the placenta. The placenta grows in your uterus and supplies your baby with food and oxygen through the umbilical cord. Some problems in the placenta can reduce the flow of blood and nutrients to your baby, which can limit your baby’s growth.
- Not gaining enough weight during pregnancy. Women who don’t gain enough weight during pregnancy are more likely to have a low-birthweight baby than women who gain the right amount of weight.
- Having a low-birthweight baby in a previous pregnancy
Risk factors in your everyday life for having a low-birthweight baby
- Smoking, drinking alcohol, using street drugs and abusing prescription drugs. Pregnant women who smoke are nearly twice as likely to have a low-birthweight baby than women who don’t smoke. Smoking, drinking alcohol, using street drugs and abusing prescription drugs during pregnancy can slow your baby’s growth in the womb and increase the risk for premature birth and birth defects.
- Having little education, low income or being unemployed
Other risk factors for having a low-birthweight baby: Age and race/ethnicity
Being younger than 17 or older than 35 makes you more likely than other women to have a low-birthweight baby. And race/ethnicity is a risk factor, too. In the United States, black women are more likely than others to have a low-birthweight baby. A little more than 13 percent of black babies are born with low birthweight each year. As for other races/ethnicities in this country, 8.4 percent of Asian babies, 7.6 percent of Native American babies, and about 7 percent of Hispanic and white babies are born with low birthweight. We don’t know why race plays a role in having a low-birthweight baby; researchers are working to learn more about it.
What health conditions can low birthweight cause for newborns?
Low-birthweight babies are more likely than babies with normal weight to have health problems as a newborn. Some need special care in a hospital’s newborn intensive care unit (also called NICU) to treat medical problems like:
- Respiratory distress syndrome (also called RDS). This breathing problem is common in babies born before 34 weeks of pregnancy. Babies with RDS don’t have a protein called surfactant that keeps small air sacs in the lungs from collapsing. Treatment with surfactant helps these babies breathe more easily. Babies with RDS also may need oxygen and other breathing help to make their lungs work.
- Bleeding in the brain (also called intraventricular hemorrhage or IVH). Bleeding in the brain can affect low-birthweight premature babies, usually in the first 3 days of life. Brain bleeds usually are diagnosed with an ultrasound. Most brain bleeds are mild and fix themselves with no or few lasting problems. More severe bleeds can cause pressure on the brain that can cause fluid to build up in the brain. This can cause brain damage. To reduce the fluid, your baby may be treated with medicine. In some cases, a surgeon may insert a tube into the baby’s brain to drain the fluid.
- Patent ductus arteriosus (also called PDA). PDA is a common heart problem for premature babies. Before birth, a large artery called the ductus arteriosus lets the baby’s blood bypass his lungs. This artery usually closes after birth so that blood can travel to the baby’s lungs and pick up oxygen. When the artery doesn’t close properly, it can lead to heart failure. Providers use tests like ultrasound to check for PDA. Babies with PDA are treated with a drug that helps close the artery. If the drug doesn’t work, a baby may need surgery.
- Necrotizing enterocolitis (also called NEC). This is a problem in a baby’s intestines. The intestines are long tubes below the stomach that help digest food. NEC usually develops 2 to 3 weeks or later after birth. It can be dangerous for a baby. It can lead to feeding problems, swelling in the belly and other complications. Babies with NEC are treated with antibiotics (medicines that kill infections) and fed intravenously (through a vein) instead of by mouth while the intestine heals. In some cases, a baby may need surgery to remove damaged parts of intestine.
- Retinopathy of prematurity (also called ROP). ROP affects blood vessels in the eye. It mostly affects babies born before 32 weeks of pregnancy. Most cases heal themselves with little or no vision loss. Some babies need treatment, though, to prevent vision loss.
What health conditions can low birthweight cause later in life?
Babies born with low birthweight may be more likely than babies born at a normal weight to have certain health conditions later in life, including:
- Heart disease
- High blood pressure
- Metabolic syndrome. This is caused when you have high blood pressure, diabetes and heart disease all together.
- Obesity. This means being very overweight. If you're obese, your body mass index (BMI) is 30 or higher. To find out your BMI, go to www.cdc.gov/bmi.
Talk to your baby’s health care provider about what you can do to help your baby be healthy. As your child grows, make sure she eats healthy food, stays active and goes to all her health care checkups. Getting regular checkups throughout childhood can help your baby’s provider spot health conditions that may cause problems as your baby grows older. These checkups also help make sure that your child gets all the vaccinations she needs to stay protected from certain harmful diseases.
Last reviewed October 2014
Frequently Asked Questions
How do I calculate adjusted age for preemies?
Chronological age is the age of a baby from the day of birth. Adjusted age is the age of the baby based on his due date. To calculate adjusted age, take your baby's chronological age (for example, 20 weeks) and subtract the number of weeks premature the baby was (6 weeks). This baby's adjusted age (20 - 6) is 14 weeks. Health care providers may use this age when they evaluate the baby's growth and development. Most premature babies catch up to their peers developmentally in 2 to 3 years. After that, differences in size or development are most likely due to individual differences, rather than to premature birth. Some very small babies take longer to catch up.
What does it mean if a baby is born “late preterm?”
Late preterm means that a baby is born after 34 weeks but before 37 weeks of pregnancy. It's important to try to have your baby as close to 39 weeks of pregnancy as possible. In the last few weeks of pregnancy, your baby's organs, like his brains, lungs and liver, are still growing. Waiting until you're at least 39 weeks also gives your baby time to gain more weight and makes him less likely to have vision and hearing problems after birth. Your baby will also be better able to suck and swallow and stay awake long enough to eat after he's born. Babies born early sometimes can't do these things.