What is a premature baby?

A premature baby, also known as a preterm baby, is a baby born before 37 weeks of gestation or pregnancy. Because premature babies have less time to grow and develop in the uterus, they are more likely to have health problems and may need special care after their birth. 

A developing baby goes through important growth throughout pregnancy including in the final months and weeks. For example, the brain, lungs, and liver need the final weeks of pregnancy to fully develop. 

In 2016, preterm birth affected about 1 of every 10 infants born in the United States. Preterm birth rates decreased from 2007 to 2014, and research by the US Centers for Disease Control and Prevention (CDC) shows that this decline is due, in part, to declines in the number of births to teens and young mothers. However, the preterm birth rate rose for the second straight year in 2016. Additionally, racial and ethnic differences in preterm birth rates remain. For example, in 2016, the rate of preterm birth among African-American women (14%) was about 50 percent higher than the rate of preterm birth among white women (9%).

Although every baby is different, the earlier in pregnancy a baby is born, the smaller and less developed it tends to be. This can lead to a greater risk of both short-term and long-term complications, some of which can be life-threatening.

Premature birth can be classified by gestational age (the number of weeks of pregnancy at which the baby was born) and birth weight.

The following tables from the Mayo Clinic 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)
 

 

 

 
This page focuses on what to expect after a baby has been born prematurely. For more information about the events leading up to a preterm birth and about the birth process itself, see our preterm labor and premature birth pages.      

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.


Care of premature babies

Depending on the health problems a premature baby has, they may require care in a specialized part of the hospital. This may be known as a newborn (or neonatal) intensive care unit (NICU), or special care nursery (SCN). Staff highly trained in the care of unwell newborns (sometimes called a neonatal care team) can provide a high level of care for your baby.

For parents and families, this can be a very difficult and stressful time. A premature baby may need to remain in hospital for weeks or, in some cases, months before being strong and well enough to go home. The more premature a baby is, the longer the stay in hospital is likely to be, but this does vary.

Parents play a key role in their baby's treatment and care. It may not be possible to hold or nurse very ill babies; however, contact and bonding time is important for recovery and development. As time goes on, the baby may be placed on a parent's bare chest for periods of time, in order to have skin-to-skin contact.

The doctors and staff caring for your baby will be able to provide you with information and answer any questions you may have relating to the level of care your baby will require. They can also explain any tests, treatments or equipment that are used and discuss any concerns you may have.

Diagnosis

A range of tests may be recommended for premature babies on a case-by-case basis. Some tests may only need to be done once, while others may need to be repeated to monitor progress.

Common tests include:

  • Monitoring breathing, heart rate and blood pressure;
  • Weight measurement to monitor how the baby is growing;
  • Monitoring feeding and output of urine and feces;
  • Urine tests;
  • Blood tests to detect a range of conditions or infections, and monitor aspects such as blood glucose and bilirubin levels;
  • Ultrasound scans of parts of the baby's body, including of the heart (echocardiography) and brain;
  • X-rays, particularly of the chest if the baby has breathing problems;
  • Magnetic resonance imaging (MRI) scans;
  • Eye examinations, and;
  • Hearing assessments.

Blood tests

During a blood test, blood can be drawn using a needle or by a finger prick. Your blood can then be analyzed to help diagnose and monitor a wide range of health conditions.

Echocardiography

The procedure in which ultrasound waves are used to create an image of the heart, to allow assessment of the heart's function as it beats.

Feces

Waste or excrement that is discharged from the bowels after digestion.

Glucose

A simple sugar found in many foods (such as fruit) that functions as a major energy source for the body.

MRI

A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.

Urine

The yellowish fluid made by the kidneys and moved via the urinary tract out of the body. It contains waste products, excess water and electrolytes.

Urine tests

A routine examination of the urine for cells, microbes, or chemicals that can indicate a range of different illnesses.

Bilirubin

The orange-yellow pigment formed in the liver by the breakdown of red blood cell proteins.

Types of treatment

Treatments for newborn premature babies will depend on their health problems, but may include:

  • Placing the baby in an incubator or radiant warmer to prevent hypothermia
  • Use of a ventilator to help the baby to breathe via a tube placed down the throat (endotracheal tube);
  • Medications;
  • An intravenous (IV) fluid line or 'drip' to supply fluids and medications;
  • A feeding tube into the mouth or nose if the baby is unable to feed;
  • Phototherapy for newborn jaundice;
  • Blood transfusions;
  • Laser treatment or cryotherapy for the eyes in severe cases of retinopathy of prematurity, and;
  • Surgery to treat various complications.

Phototherapy is usually very effective in the treatment of jaundice. 

Cryotherapy

The application of liquid nitrogen either by using a gun-like instrument or a swab dipped into the liquid nitrogen to freeze the affected area. It is commonly used to treat warts but can also be used to treat other conditions including trapped nerves, retinal conditions, benign skin tags and precancerous lesions.

Hypothermia

A serious condition that occurs when the body temperature falls too low, usually due to exposure to cold and damp conditions. Symptoms include uncontrollable shivering, drowsiness and confusion. It requires emergency medical treatment.

Retinopathy of prematurity

A condition of the eye seen in premature babies in which the blood vessels in the retina do not develop correctly. In severe cases it can lead to vision loss.

Transfusions

The process of receiving blood or blood components from an external source directly into the bloodstream.

Common short-term complications

Not all premature babies will experience significant complications after birth, and the types and severity of health problems will vary between babies of similar birth weights and levels of prematurity.

Newborn jaundice

Newborn jaundice is caused by excessive levels of bilirubin in a baby's blood. Bilirubin is a substance formed by the breakdown of red blood cells in the liver. It can cause yellowness of the skin and eyes, and even brain damage at very high levels, although this is rare.

Jaundice is common in all newborns, but can be more severe in premature babies. This is because they are smaller and have immature livers that are less able to break down bilirubin, leading to higher levels in the blood.

Anemia

Anemia occurs when not enough red blood cells are produced in the body. It is normal for red blood cell numbers to drop in babies in the months after they are born, but this tendency can be greater in premature babies.

Feeding problems

Premature babies may be less able to suckle and swallow, and so may need to be fed by a tube in order to get sufficient nourishment.

Unstable blood glucose levels

Glucose is an important source of energy in the body. It is common for newborns to experience some degree of fluctuation in their blood glucose levels after birth, as they adapt to getting energy through feeding, rather than from their mother's blood supply. However, premature babies are at greater risk of these problems, particularly low blood glucose levels, known as hypoglycemia. This is because their livers tend to be immature and they may have lower stored levels of another energy source, called glycogen (a storage form of glucose).

If hypoglycemia persists, it can be life-threatening, so babies with low blood glucose levels are monitored and treated if necessary.

Hypothermia

Premature babies tend to have low levels of body fat and reduced development in areas of the brain that control temperature regulation, so they are less able to shiver and produce body heat. This can lead to hypothermia, a condition in which the body temperature drops too low and the baby has difficulty staying warm and controlling body temperature.

If this is not prevented, it can cause complications such as uncontrolled blood glucose levels and in severe cases, death. Also, a baby that is constantly cold will tend to use up most of its energy feeding and keeping warm, rather than for growing and getting stronger.

A humidicrib can help premature babies attain a healthy body temperature.Premature babies can be placed into an incubator, known as a humidicrib, to help maintain a healthy body temperature. 

Increased risk of infection

Due to their immature immune systems, premature babies are at greater risk of developing an infection, which can be life-threatening if it spreads to the blood (sepsis). Infections can increase the chance of long-term complications, such as problems with growth and brain development.

Retinopathy of prematurity

Retinopathy of prematurity is a condition of the eyes that can develop because the retina, the light-sensitive layer at the back of the eye, has not developed sufficiently by birth. If the outer retina does not have adequate blood supply, abnormal blood vessels may grow that are prone to leaking, which can cause scarring.

In severe cases, the retina can be pulled out of place, leading to separation from the back of the eye and permanent vision loss or blindness.

Respiratory complications

Premature babies may also have underdeveloped lungs, which can lead to problems with breathing and getting enough oxygen into the body.

For example, respiratory distress syndrome is more common in babies born before 35 weeks' gestation compared to full-term babies. It occurs because the lungs have not produced enough of a chemical called surfactant, which allows the lungs to expand when breathing in air.

Babies born before 32 weeks are also more likely to develop chronic or long-term lung disease, also known as bronchopulmonary dysplasia.

The part of the brain that controls breathing can also be immature, leading to a condition called apnea of prematurity, where a baby will stop breathing for periods of time (seconds). The more premature a baby is, the more likely they are to experience this problem, which can reduce vital oxygen intake into the body (called hypoxia) and place increased stress on the heart.

Heart complications

Premature babies are at increased risk of conditions that affect the heart and blood vessels.

The ductus arteriosus is a blood vessel that allows blood to bypass the lungs while a baby is in the uterus and not breathing. It connects two major blood vessels of the heart and usually closes off a few days after birth, when it is no longer needed.

Patent ductus arteriosus occurs when the ductus arteriosus does not close. It is more likely to occur in babies born before 30 weeks' gestation. Depending on how large the opening is, it can lead to damage of the heart and high blood pressure in the blood vessels of the lungs, as well as other complications.

Premature babies can also be more prone to low blood pressure (hypotension), which can reduce blood flow around the body and contribute to complications such as brain hemorrhage.

Brain hemorrhage

Extremely premature babies (born before 28 weeks) are at increased risk of intraventricular hemorrhage, which is bleeding within the ventricle system of the brain.

This type of hemorrhage can often be mild and get better, but large bleeds can lead to permanent damage or development of fluid on the brain (hydrocephalus). An operation may be necessary to relieve symptoms of an intraventricular hemorrhage.

Digestive system complications

Necrotizing enterocolitis is a condition that causes parts of the bowel to die. It is a very serious condition that can be life-threatening. It can occur in full-term babies, but is much more common in premature babies, particularly those with an extremely low birth weight (less than 1000g).

It tends to occur once babies start feeding and is much more common in babies that are formula-fed, compared to babies that are breastfed.

Anemia

A deficiency in red blood cells or hemoglobin in the body.

Glucose

A simple sugar found in many foods (such as fruit) that functions as a major energy source for the body.

Hemorrhage

Excessive bleeding that is difficult to stop.

Hypothermia

A serious condition that occurs when the body temperature falls too low, usually due to exposure to cold and damp conditions. Symptoms include uncontrollable shivering, drowsiness and confusion. It requires emergency medical treatment.

Immune systems

The organs and cells involved in protecting the body against infection.

Infection

Entry into the body of microorganisms that can reproduce and cause disease.

Red blood cells

Cells in the blood that transport oxygen from the lungs throughout the body and carbon dioxide from the body to the lungs.

Retinopathy of prematurity

A condition of the eye seen in premature babies in which the blood vessels in the retina do not develop correctly. In severe cases it can lead to vision loss.

Uterus

The hollow organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. Also known as the womb.

Bilirubin

The orange-yellow pigment formed in the liver by the breakdown of red blood cell proteins.

Ventricle

1. The two main heart chambers that eject blood back out of the heart. 2. A series of connecting cavities in the brain that contain the cerebrospinal fluid, which supports and protects the brain.

Bronchopulmonary dysplasia

A long-term condition characterized by abnormal development of the lung tissue.

Common long-term complications

Premature babies can be at increased risk of long-term complications that can affect them throughout their lives. They are more likely to need follow-up hospital visits and may have ongoing problems with long-term health and developmental issues.

Children who were born prematurely may be at increased risk of:

Breathing problems

Breathing problems related to premature birth can include asthma and various infections, such as those of the lower respiratory tract . Children with bronchopulmonary dysplasia are particularly prone to serious breathing problems and are more likely to require readmission to hospital for treatment.

Cerebral palsy

Cerebral palsy is a group of conditions that affect how a person moves and controls their muscles. It can be caused by damage to the brain before birth, or in the first few months after birth.

Developmental problems

Some premature babies may be slower to reach developmental milestones and have problems with movements and/or fine motor skills. They are also at greater risk of learning disabilities, which may not become apparent until they begin school. This may require extra support at home and in school.

Psychological issues

Premature birth is linked to a range of behavioral and mental health issues, including autism, ADHD, depression, generalized anxiety disorder and a short attention span.

Eye problems

Eye problems that may occur in connection with premature birth include strabismus, amblyopia, myopia (shortsightedness) and astigmatism. People with retinopathy of prematurity are also at increased risk of developing retinal detachment and glaucoma.

Hearing loss

Hearing loss can occur because of damage to the ear or brain. Babies cared for in a neonatal intensive care unit (NICU) are generally screened for hearing loss before they go home from hospital, but later assessment may also be recommended to detect problems as the child grows.

Adult health issues

As adults, people who were born prematurely may be at greater risk of developing high blood pressure and insulin resistance that can lead to type 2 diabetes.

Amblyopia

Lazy eye. A condition that arises in children in which the vision in one eye does not develop as well as the other eye. Often associated with strabismus (turned eye) and may be treated by patching.

Astigmatism

A refractive error of the eye in which the cornea or lens inside the eye are irregularly shaped and do not focus light equally on the back of the eye. It can cause blurred vision.

Retinal detachment

A condition in which part or all of the retina becomes detached from the back of the eye. Retinal detachment can lead to serious damage to the retina and permanent vision loss.

Retinopathy of prematurity

A condition of the eye seen in premature babies in which the blood vessels in the retina do not develop correctly. In severe cases it can lead to vision loss.

Strabismus

Also known as 'cross-eyed'. It is a condition where the eyes are not properly aligned and look in different directions.

Bronchopulmonary dysplasia

A long-term condition characterized by abnormal development of the lung tissue.

Prognosis

The prognosis for premature babies varies widely and very much depends on how early they are born, their birth weight and any complications that develop. While some premature babies will grow up with few or no lasting complications, others will experience a range of long-term health problems. Unfortunately, premature babies have a much higher rate of death in the months after birth than full-term babies. This is particularly the case for those that are born very prematurely. 

Further references

Preterm Birth | Maternal and Infant Health | Reproductive Health | CDC

Premature birth - Symptoms and causes - Mayo Clinic