A premature birth means a baby is born too early. The birth takes place before the 37th week of pregnancy. A typical pregnancy lasts about 40 weeks.

Premature babies often have serious health problems, especially when they're born very early. These problems often vary. But the earlier a baby is born, the higher the risk of health challenges.

A newborn can be:

  • Late preterm, born between 34 and 36 completed weeks of pregnancy.
  • Moderately preterm, born between 32 and 34 weeks of pregnancy.
  • Very preterm, born between 28 and 32 weeks of pregnancy.
  • Extremely preterm, born before 28 weeks of pregnancy.

Most premature births happen in the late preterm stage.


Your baby may have very mild symptoms of premature birth or more-serious health problems.

Some signs of being born too early include:

  • Small size, with a head that's large compared with the body.
  • Features that are sharper and less rounded than a full-term baby's features due to a lack of cells that store fat.
  • Fine hair that covers much of the body.
  • Low body temperature, mainly right after birth in the delivery room.
  • Trouble breathing.
  • Feeding problems.

The following tables show the median birth weight, length and head circumference of premature babies at different gestational ages for each sex.

Weight, length and head circumference by gestational age for boys
Gestational age Weight Length Head circumference
40 weeks 7 lbs., 15 oz.
(3.6 kg)
20 in. (51 cm) 13.8 in. (35 cm)
35 weeks 5 lbs., 8 oz.
(2.5 kg)
18.1 in. (46 cm) 12.6 in. (32 cm)
32 weeks 3 lbs., 15.5 oz.
(1.8 kg)
16.5 in. (42 cm) 11.6 in. (29.5 cm)
28 weeks 2 lbs., 6.8 oz.
(1.1 kg)
14.4 in. (36.5 cm) 10.2 in. (26 cm)
24 weeks 1 lb., 6.9 oz.
(0.65 kg)
12.2 in. (31 cm) 8.7 in. (22 cm)
Weight, length and head circumference by gestational age for girls
Gestational age Weight Length Head circumference
40 weeks 7 lbs., 7.9 oz.
(3.4 kg)
20 in. (51 cm) 13.8 in. (35 cm)
35 weeks 5 lbs., 4.7 oz.
(2.4 kg)
17.7 in. (45 cm) 12.4 in. (31.5 cm)
32 weeks 3 lbs., 12 oz.
(1.7 kg)
16.5 in. (42 cm) 11.4 in. (29 cm)
28 weeks 2 lbs., 3.3 oz.
(1.0 kg)
14.1 in. (36 cm) 9.8 in. (25 cm)
24 weeks 1 lb., 5.2 oz.
(0.60 kg)
12.6 in. (32 cm) 8.3 in. (21 cm)

Special care

If you give birth to a preterm baby, your baby will likely need to stay in a special nursery unit at the hospital. Some infants need to spend time in a unit that cares for them and closely tracks their health day and night. This is called a neonatal intensive care unit (NICU).

A step down from the NICU is an intermediate care nursery, which provides less intensive care. Special nursery units are staffed with health care providers and a team that's trained to help preterm babies.

Your baby may need extra help feeding and adapting right after delivery. Your health care team can help you understand what's needed and what your baby's care plan will be. Feel free to ask them questions.

Risk factors

Often, the exact cause of premature birth isn't clear. But certain things can raise the risk.

Some risk factors linked to past and present pregnancies include:

  • Pregnancy with twins, triplets or other multiples.
  • A span of less than six months between pregnancies. It's ideal to wait 18 to 24 months between pregnancies.
  • Treatments to help you get pregnant, called assisted reproduction, including in vitro fertilization.
  • More than one miscarriage or abortion.
  • A previous premature birth.

Some health problems can raise the risk of premature birth, such as:

  • Problems with the uterus, cervix or placenta.
  • Some infections, mainly those of the amniotic fluid and lower genital tract.
  • Ongoing health problems such as high blood pressure and diabetes.
  • Injuries or trauma to the body.

Lifestyle choices also can raise the risk of a preterm pregnancy, such as:

  • Smoking cigarettes, taking illicit drugs or drinking alcohol often or heavily while pregnant.
  • Being underweight or overweight before pregnancy.
  • Becoming pregnant before the age of 17 or after 35.
  • Going through stressful life events, such as the death of a loved one or domestic violence.

For unknown reasons, Black and Native people in the United States are more likely to have premature births than are women of other races. But premature birth can happen to anyone. In fact, many preterm births have no known risk factors.


Not all premature babies have health complications. But being born too early can cause short-term and long-term medical problems. In general, the earlier a baby is born, the higher the risk of complications. Birth weight plays a key role too.

Some problems may be clear at birth. Others may not show up until later.

Short-term complications

In the first weeks, the complications of premature birth may include:

  • Breathing problems. A premature baby may have trouble breathing due to being born with lungs that aren't fully developed. If the baby's lungs lack a substance that allows the lungs to expand, the baby may have trouble getting enough air. This is a treatable problem called respiratory distress syndrome.

    It's common for preterm babies to have pauses in their breathing called apnea. Most infants outgrow apnea by the time they go home from the hospital. Some premature babies get a less common lung disorder called bronchopulmonary dysplasia. They need oxygen for a few weeks or months, but they often outgrow this problem.

  • Heart problems. Some common heart problems that premature babies have are patent ductus arteriosus (PDA) and low blood pressure. PDA is an opening between two important blood vessels, the aorta and the pulmonary artery. This heart defect often closes on its own. But without treatment it can lead to problems such as heart failure. That's when the heart can't pump blood as well as it should. Low blood pressure may need to be treated with fluids given through a vein, medicines and sometimes blood transfusions.
  • Brain problems. The earlier a baby is born, the greater the risk of bleeding in the brain. This is called an intraventricular hemorrhage. Most hemorrhages are mild and resolve with little short-term impact. But some babies may have larger brain bleeding that causes permanent brain injury.
  • Temperature control problems. Premature babies can lose body heat quickly. They don't have the stored body fat of a full-term infant. And they can't make enough heat to counter what's lost through the surface of their bodies. If body temperature drops too low, it can lead to a dangerous problem called hypothermia.

    Hypothermia in a premature baby can lead to breathing problems and low blood sugar levels. A preterm infant also may use up all of the energy gained from feedings just to stay warm. That's why smaller premature babies need extra heat from a warmer or an incubator at first.

  • Digestive problems. Premature infants are more likely to have digestive systems that aren't fully developed. This can lead to problems such as necrotizing enterocolitis (NEC). With NEC, the cells lining the bowel wall are injured. This problem can happen in premature babies after they start feeding. Premature babies who receive only breast milk have a much lower risk of getting NEC.
  • Blood problems. Premature babies are at risk of blood problems such as anemia and newborn jaundice. With anemia, the body doesn't have enough red blood cells. All newborns have a slow drop in red blood cell count during the first months of life. But that drop may be greater in premature babies. With newborn jaundice, the skin and eyes look yellow. It happens because the baby's blood contains too much of a yellow-colored substance from the liver or red blood cells. This substance is called bilirubin. Jaundice has many causes, but it is more common in preterm babies.
  • Metabolism problems. Premature babies often have problems with metabolism. That's the process by which the body changes food and drink into energy. Some premature babies may have a very low level of blood sugar. This can happen because premature infants often have smaller amounts of stored blood sugar than do full-term babies. Premature babies also have more trouble turning their stored sugar into more-usable, active forms of blood sugar.
  • Immune system problems. It's common for premature babies to have immune systems that aren't fully developed. This can lead to a higher risk of illnesses. An infection in a premature baby can quickly spread to the bloodstream and cause a life-threatening problem called sepsis.

Long-term complications

Over the long term, premature birth may lead to health problems such as:

  • Cerebral palsy. This group of disorders can cause problems with movement, muscle tone or posture. It can be due to an infection or poor blood flow. It also can stem from an injury to a newborn's brain, either early during pregnancy or while the baby is still young.
  • Trouble learning. Premature babies are more likely to lag behind full-term babies on different milestones. A school-age child who was born too early might be more likely to have learning disabilities.
  • Vision problems. Premature infants may get an eye disease called retinopathy of prematurity. This happens when blood vessels swell and grow too much in the light-sensing tissue at the back of the eye, called the retina. Sometimes these overgrown vessels slowly scar the retina and pull it out of place. When the retina is pulled away from the back of the eye, it's called retinal detachment. Without treatment, this can harm vision and cause blindness.
  • Hearing problems. Premature babies have a higher risk of losing some hearing. All babies should have their hearing checked before they go home from the hospital.
  • Dental problems. Preterm babies may be more likely than full-term babies to have defects with the hard outer covering of the teeth, called enamel. Infants born very or extremely early also may be more likely to have teeth that take longer to develop.
  • Behavior and mental health problems. Children who were born early may be more likely than kids born full term to have certain mental health troubles, as well as delays in development.
  • Ongoing health issues. Premature babies are more likely to have long-term health issues than are full-term infants. Illnesses, asthma and feeding problems are more likely to develop or linger. Premature infants also are at higher risk of sudden infant death syndrome (SIDS). That's when an infant dies for unclear reasons, often while asleep.


The exact cause of preterm birth is often unknown. But some things can be done to help lower the risk of preterm birth, including:

  • Taking progesterone supplements. Progesterone is a hormone that plays a role in pregnancy. A lab-made version of it may be able to lower the risk of preterm birth if you've had a premature baby before. It also may lower the risk of preterm birth if you have a short cervix. The cervix is the lower end of the uterus, which opens during labor so a baby can be born.
  • Cervical cerclage. This is a surgery that's done during pregnancy. Your provider may suggest it if you have a short cervix and you've had a preterm birth before.

    During this procedure, the cervix is stitched closed with a strong suture. This may give the uterus extra support. The suture is removed when it's time to have the baby. Ask your provider if you need to stay away from vigorous activity during the rest of your pregnancy.

One thing that doesn't help prevent preterm birth is staying in bed. Bed rest can raise the risk of blood clots, weaker bones and less muscle strength. It might even make preterm birth more likely.

March 22, 2024
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