At birth, a baby is classified as one of the following:

• Premature (less than 37 weeks gestation)
• Full term (37 to 42 weeks gestation)
• Post term (born after 42 weeks gestation)

If a woman goes into labor before 37 weeks, it is called preterm labor.

Late preterm babies who are born between 35 and 37 weeks gestation may not look premature. They may not be admitted to a neonatal intensive care unit (NICU), but they are still at risk for more problems than full-term babies.

Health conditions in the mother, such as diabetes, heart disease, and kidney disease, may contribute to preterm labor. Often, the cause of preterm labor is unknown. Some premature births are multiple pregnancies, such as twins or triplets.

Different pregnancy-related problems increase the risk of preterm labor or early delivery:

• A weakened cervix that begins to open (dilate) early, also called cervical incompetence
• Birth defects of the uterus
• History of preterm delivery
• Infection (a urinary tract infection or infection of the amniotic membrane)
• Poor nutrition right before or during pregnancy
• Preeclampsia: high blood pressure and protein in the urine that develop after the 20th week of pregnancy
• Premature rupture of the membranes (placenta previa)

Other factors that increase the risk for preterm labor and a premature delivery include:

• Age of the mother (mothers who are younger than 16 or older than 35)
• Being African American
• Lack of prenatal care
• Low socioeconomic status
• Use of tobacco, cocaine, or amphetamines

The infant may have trouble breathing and keeping a constant body temperature.

A premature infant may have signs of the following problems:

• Not enough red blood cells (anemia)
• Bleeding into the brain or damage to the brain's white matter
• Infection or neonatal sepsis
• Low blood sugar (hypoglycemia)
• Neonatal respiratory distress syndrome, extra air in the tissue of the lungs (pulmonary interstitial emphysema), or bleeding in the lungs (pulmonary hemorrhage)
• Yellow skin and whites of the eyes (newborn jaundice)
• Problems breathing due to immature lungs, pneumonia, or patent ductus arteriosus
• Severe intestinal inflammation (necrotizing enterocolitis)

A premature infant will have a lower birth weight than a full-term infant. Common signs of prematurity include:

• Abnormal breathing patterns (shallow, irregular pauses in breathing called apnea)
• Body hair (lanugo)
• Enlarged clitoris (in female infants)
• Less body fat
• Lower muscle tone and less activity than full-term infants
• Problems feeding due to trouble sucking or coordinating swallowing and breathing
• Small scrotum that is smooth and has no ridges, and undescended testicles (in male infants)
• Soft, flexible ear cartilage
• Thin, smooth, shiny skin that is often transparent (can see veins under skin)

Common tests performed on a premature infant include:

• Blood gas analysis to check oxygen levels in the blood
• Blood tests to check glucose, calcium, and bilirubin levels
• Chest x-ray
• Continuous cardiorespiratory monitoring (monitoring of breathing and heart rate)

When premature labor develops and can't be stopped, the health care team will prepare for a high-risk birth. The mother may be moved to a center that is set up to care for premature infants in a NICU.

After birth, the baby is admitted to the NICU. The infant is placed under a warmer or in a clear, heated box called an incubator, which controls the air temperature. Monitoring machines track the baby's breathing, heart rate, and level of oxygen in the blood.

A premature infant's organs are not fully developed. The infant needs special care in a nursery until the organs have developed enough to keep the baby alive without medical support. This may take weeks to months.

Infants usually cannot coordinate sucking and swallowing before 34 weeks gestation. A premature baby may have a small, soft feeding tube placed through the nose or mouth into the stomach. In very premature or sick infants, nutrition may be given through a vein until the baby is stable enough to receive all nutrition through the stomach.

If the infant has breathing problems:

• A tube may be placed into the windpipe (trachea). A machine called a ventilator will help the baby breathe.
• Some babies whose breathing problems are less severe receive continuous positive airway pressure (CPAP) with small tubes in the nose instead of the trachea. Or they may receive only extra oxygen.
• Oxygen may be given by ventilator, CPAP, nasal prongs, or an oxygen hood over the baby's head.

Infants need special nursery care until they are able to breathe without extra support, eat by mouth, and maintain body temperature and body weight. Very small infants may have other problems that complicate treatment and require a longer hospital stay.

There are many support groups for parents of premature babies. Ask the social worker in the neonatal intensive care unit.

Prematurity used to be a major cause of infant deaths. Improved medical and nursing techniques have increased the survival of premature infants.

Prematurity can have long-term effects. Many premature infants have medical, developmental, or behavioral problems that continue into childhood or are permanent. The more premature the baby is and the smaller their birth weight is, the greater the risk for complications. However, it is impossible to predict a baby's long-term outcome based on gestational age or birth weight.

Possible long-term complications include:

• Long-term lung problem called bronchopulmonary dysplasia (BPD)
• Delayed growth and development
• Mental or physical disability or delay
• Vision problem called retinopathy of prematurity, resulting in low vision or blindness

The best ways to prevent prematurity are to:

• Be in good health before getting pregnant.
• Get prenatal care as early as possible in the pregnancy.
• Continue to get prenatal care until the baby is born.

Getting early and good prenatal care reduces the chance of premature birth.

Premature labor can sometimes be treated or delayed by a medicine that blocks uterine contractions. Many times, however, attempts to delay premature labor are not successful.

Betamethasone (a steroid medicine) given to mothers in premature labor can make some prematurity complications less severe.

Preterm infant; Preemie; Premie; Neonatal - premie; NICU - premie

Brady JM, Barnes-Davis ME, Poindexter BB. The high-risk infant. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 117.

Parsons KV, Jain L. The late preterm infant. In: Martin RJ, Fanaroff AA, Walsh MC, eds. Faranoff and Martin's Neonatal-Perinatal Medicine. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 40.

Simhan HN, Romero R. Preterm labor and birth. In: Landon MB, Galan HL, Jauniaux ERM et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 36.

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.