Premature birth, also known as preterm birth, is when a baby is born earlier than usual, before 37 weeks of pregnancy.

Because premature babies are born early, they have not had as much time as usual in the womb to develop and grow. As a result, they are at risk for a number of health problems, in large part because their organs—such as the brain, lungs, liver, and kidneys, among others—may be underdeveloped. The types of health problems vary, but they may include difficulty maintaining body temperature, breathing, and feeding, as well as developmental delay, and vision and hearing problems, among others. Though many of these health problems can be temporary, in some cases they are long-term and can have lifelong consequences. The earlier in pregnancy a baby is delivered, the higher the risk for these and other health problems.

In the United States, premature infants make up around 1 out of every 10 births. The majority of premature infants in the U.S. are born between 34 and 37 weeks of pregnancy.

While premature babies—in particular those who are born extremely prematurely—are at increased risk for a number of health problems, most do not have any long-term health issues related to an early birth.

“Neonatologists are doctors who specialize in health care for preterm infants, and they deliver this care in the neonatal intensive care unit [NICU], says Sarah N. Taylor, MD, chief of Yale Neonatal Perinatal Medicine. “Although preterm birth is associated with increased health risks for the infant, neonatal research continues to discover ways to improve preterm infant health care and decrease the risk for short-term and long-term problems.”

A premature birth is when a baby is born early, before the 37^th week of pregnancy. Babies born before the 37^th week of pregnancy are referred to as “premature” or “preterm” infants or babies. Normal pregnancies last between 37 and 42 weeks.

Health care providers often categorize premature infants based on the gestational age (the duration of a pregnancy as measured from the first day of the last menstrual period to the current day) at delivery, as follows:

• Late preterm: between 34 and less than 37 weeks
• Moderately preterm: between 32 weeks and less than 34 weeks
• Very preterm: between 28 and less than 32 weeks
• Extremely preterm: less than 28 weeks

They may also be classified based on their birth weight, as follows:

• Low birth weight: less than 2,500 grams (about 5.5 pounds)
• Very low birth weight: less than 1,500 grams (about 3.3 pounds)
• Extremely low birth weight: less than 1,000 grams (2.2 pounds)

The more premature a baby is at birth, the higher the risk for serious and long-term health problems, and, in some cases, death.

The majority of premature births are spontaneous, meaning they are unexpected and unplanned. Less frequently, preterm deliveries are planned due to a known medical problem or complication in the pregnant person, placenta, or fetus, such as an infection or preeclampsia. In these cases, a health care provider may induce early labor or perform a C-section delivery.

The causes of premature birth are often unknown. Certain factors, however, can increase the risk for a preterm birth, including the following:

Risk factors associated with a previous pregnancy:

• Previous preterm birth (a significant risk factor for a premature birth)

Risk factors associated with the current pregnancy:

• Multiple pregnancy (i.e., being pregnant with twins, triplets, or more)
• Use of assisted reproductive technologies in the current pregnancy (e.g., in vitro fertilization)
• Short time interval between pregnancies (less than 6 months between pregnancies)
• Undernutrition
• Pregestational and gestational diabetes
• Preeclampsia (new onset of high blood pressure and excess protein in the urine after the 20^th week of pregnancy)
• Having a short cervix or if the cervix shortens during the second trimester rather than the third trimester
• Vaginal bleeding in the first trimester
• Placenta previa (when the placenta attaches to the lowest part of the uterus and covers part or all of the cervix)
• Placental abruption (when the placenta detaches from the uterus before delivery)
• Infections, including:
  • Urinary tract infection
  • Sexually transmitted infection
  • Intraamniotic infection (or chorioamnionitis), an infection of the membranes surrounding the fetus, the placenta, the amniotic fluid, and/or the fetus
  • Bacterial vaginosis (an infection caused by change in the balance of bacteria in the vagina)
• Preterm premature rupture of the membranes (PPROM), when the amniotic sac (the fluid-filled membranes surrounding the fetus) ruptures prior to the 37^th week of pregnancy before labor begins
• Certain congenital (present at birth) defects
• Gaining too little or too much weight during pregnancy
• Lack of prenatal care

Other risk factors:

• Certain drugs, medications, and toxins, including:
  • Cigarette smoking
  • Alcohol use
  • Illicit drug use
  • Certain medications (e.g., beta blockers)
  • Exposure to some environmental pollutants
• Younger or older age of pregnant person (those under age 18 or over 35 are at increased risk)
• Uterine fibroids (noncancerous growths in the uterus)
• Certain chronic health conditions, including:
  • Diabetes
  • High blood pressure
  • Kidney disease
• Stress
• Depression
• Being underweight or obese before becoming pregnant
• Long working hours (e.g., over 80 hours/week) or work involving long periods of standing
• Domestic violence
• Lack of social support

Most complications in premature infants occur because certain organs have not yet fully developed and, as a result, do not work properly. The severity of these effects tends to increase the more prematurely a baby is born.

Complications may include:

• Lungs issues. Premature infants with underdeveloped lungs may have trouble breathing. Breathing difficulty may occur in premature infants who do not produce enough surfactant, a slippery substance composed of fats and proteins that keep the air sacs (called alveoli) in the lungs open and prevents them from deflating while breathing. Without enough surfactant, the lungs may collapse, making breathing difficult. This condition is known as respiratory distress syndrome, or RDS.
  
  Some premature babies, especially those who are very premature and have an increased risk of RDS, may need breathing support using a mechanical ventilator, nasal prongs, or cannulae. However, because the lungs of premature babies can be underdeveloped and fragile, the oxygen and pressure from these breathing supports can damage the lungs and nearby blood vessels, causing breathing problems and resulting in a condition called bronchopulmonary dysplasia (BPD). BPD may cause longer-term issues with breathing, and some affected infants may require prolonged breathing support or medications.

• Heart issues. Premature babies may also have heart problems, including patent ductus arteriosus, in which a blood vessel near the heart that normally closes shortly after birth does not close. Over time, this can lead to heart failure, pulmonary hypertension, and other problems. Some patients may need to have this blood vessel closed, either with medication or through a surgical procedure.
• Eyes issues. Premature infants may also have problems with their eyes, including:
• Retinopathy of prematurity (ROP), a condition in which blood vessels of the retina (a layer of tissue at the back of the eye) do not develop properly and may grow in the wrong direction, possibly pulling the retina away from the back of the eye (known as retinal detachment). In severe cases, ROP can result in vision loss or blindness.
• Nearsightedness (myopia), eyes that are not aligned with one another (strabismus), or both.
• Blood issues. Babies born very early often lack the ability to produce red blood cells in adequate numbers, and necessary blood tests can further deplete their already less than optimal blood supply. Thus, significant anemia can develop in the early days and weeks of life.
• Brain issues. The brains of premature infants may be underdeveloped. This can result in a number of health problems, including:
• Apnea of prematurity, pauses in breathing that last 20 seconds or longer in premature babies, may occur if the parts of the brain that control breathing are not fully developed. Pauses in breathing may cause the heartrate to slow down temporarily. Some babies require medication or breathing support until their breathing control centers have matured.
• Poor coordination of sucking and swallowing reflexes can occur if certain parts of the brain are underdeveloped. This typically affects infants born before 34 weeks of gestation. The poor coordination impairs the ability to suck and swallow, and as a result, infants with this problem must be fed via a feeding tube.
• Bleeding in the brain (intraventricular hemorrhage) may occur if blood vessels in the brain are underdeveloped.
• Developmental delay and/or cognitive delay may also occur if the brain is underdeveloped. Further, in the long term, premature babies can also be at a greater risk for complications including cognitive delays, hearing problems, behavioral issues, and sometimes, cerebral palsy.
• Problems regulating body temperature. Many premature babies have difficulty maintaining their body temperature, and must be kept warm to prevent their temperature from dropping too low (known as hypothermia). Some babies require help with temperature regulation using an incubator.
• Immune system issues. Because the immune system is underdeveloped, premature babies are at increased risk for infections.
• Kidneys issues. The kidneys of premature infants may be underdeveloped, which can result in problems regulating water, salt, and mineral levels in the blood. The kidneys are responsible for removing waste and acid from the body. Since premature infants have underdeveloped kidneys, too much acid may build up in the body (known as metabolic acidosis).
• Gastrointestinal tract issues. Premature infants with underdeveloped digestive tracts may have difficulties digesting food and may frequently spit up food and acid from the stomach (known as gastroesophageal reflux). Some premature infants need to receive their nutrition intravenously. Premature infants may also be at risk for necrotizing enterocolitis (NEC), a serious condition impacting the intestines. NEC can be life-threatening, and affected infants may require antibiotics, surgery, or additional support for breathing and blood pressure.
• Liver issues. Among many other functions, the liver removes bilirubin from the blood. Bilirubin is a yellowish substance produced during the normal process of breaking down old red blood cells. If the liver is underdeveloped, bilirubin can build up in the blood (known as hyperbilirubinemia), resulting in jaundice (yellowing of the skin and eyes). Very high levels of bilirubin can impact the brain, so some patients require treatments to decrease these levels, including phototherapy (exposure to a special blue light), medication, or an exchange transfusion (where blood with a high bilirubin level is removed and replaced).
• Endocrine issues: Premature babies may also have difficulties regulating glucose levels in their blood and may have low (hypoglycemia) or high (hyperglycemia) blood sugar levels.
• Bone issues: Development can also be impacted by prematurity, increasing risk for fractures.

Premature infants born before the 35^th week of pregnancy are typically treated in a neonatal intensive care unit (NICU), where health care providers provide around-the-clock care. Premature infants born between 35 and 37 weeks of pregnancy may be cared for in the well newborn nursery, and usually have a shorter hospital stay than more premature infants.

Treatment for premature infants involves addressing problems related to underdeveloped organs. Treatment may include:

• Incubator. After birth, premature infants may be placed in a heated incubator to help them keep warm.
• Monitoring. Premature infants are connected to machines that monitor vital signs including heart rate, blood pressure, breathing, and blood oxygen level.
• Breathing support. Premature infants with breathing difficulties may need some type of breathing support to provide additional oxygen and/or pressure. This may involve a mechanical ventilator, which involves inserting a tube into the baby’s windpipe. A continuous positive airway pressure (CPAP) machine may also be an option. CPAP uses air pressure to keep the airways open. In CPAP, small prongs or cannulae may be inserted into the baby’s nose.
• Surfactant. Premature infants with RDS may be given a medicine called surfactant to help keep the air sacs in their lungs open, thereby improving breathing.
• Feeding assistance. Because premature infants may have difficulty coordinating sucking and swallowing, as well as an underdeveloped digestive tract, they may require feeding assistance. Sometimes, this may be accomplished via a feeding tube that is inserted through the nose or mouth and guided into the stomach. In other cases, nutrition may need to be given intravenously.
• Blood transfusions. Premature babies with anemia or low levels of clotting cells or factors may need blood transfusions.

Additional treatments may be necessary depending on the complications and needs of each premature baby.

Parents can visit their baby in the NICU, and skin-to-skin contact is encouraged. In some institutions, parents can stay with their baby in the NICU.

Premature babies may be discharged from the hospital when they are able to perform certain functions on their own, including:

• Maintain body temperature
• Breastfeed or feed from a bottle, and take in enough nutrition to gain weight
• Weigh enough to safely fit into an infant car seat
• No longer have episodes of apnea and slow heartbeat
• Safely sleep on their back

Any other health problems must be effectively managed before they can go home.

Infants born more prematurely typically need to spend more time in the hospital than later preterm infants. Hospital discharge decisions are often made based on whether the infant meets the functional criteria mentioned above rather than by the infant’s gestational age. While the age at hospital discharge can vary greatly, many infants are able to go home when they reach 35 to 37 weeks of gestational age.

After leaving the hospital, premature infants typically require follow-up care to monitor their development and to receive any additional treatment or support they need.

The prognosis for premature infants varies based on a number of factors, including their degree of prematurity, birth weight, sex, and access to specialized medical care, among other factors. In general, late preterm babies (those born between 34 and 37 weeks) have a better outlook than those born more prematurely. Similarly, those born with higher birth weights tend to have better outlook than those with lower birth weights. Male premature infants are at increased risk for certain serious health problems, including functional disability due to neurologic issues, than female premature infants with the same degree of prematurity.

A neonatal intensive care unit (NICU) has specialized equipment and treatments to care for premature or ill infants. Treatment advancements have improved the survival rate of premature infants, including those born extremely preterm (before the 28^th week of pregnancy). For instance, in the 1960s, premature infants with RDS had a 5% survival rate; by 2022, the survival rate had increased to approximately 90%. Even with advances in treatment and improved survival rates, premature infants can still have health problems that last for years or may even be permanent. Most premature babies, though, have a positive outlook and do not have severe long-term disabilities.

Yale Medicine has been recognized as a national leader in the prevention of hospital-acquired infections in the NICU, having won multiple awards for this work.

The care of premature babies after discharge at Yale Medicine is coordinated in the NICU GRAD (“great results after discharge”) Program, directed by Angela Montgomery, MD, MSEd.

“The outlook for premature babies has continued to improve with advances in medical care. While not all negative outcomes of prematurity can be prevented, a focus on comprehensive, family-centered, longitudinal care during a pregnancy, through the neonatal intensive care unit stay, and following hospital discharge continues to lead to improved outcomes,” says Angela Montgomery, MD, MSEd, director of Yale NICU GRAD follow-up program.