Preterm birth is a major challenge in perinatal health care. It is a leading cause of perinatal death and is associated with neurological impairment and disability. Preterm infants require prolonged hospitalization and have significant cost implications for health services. Preterm birth is defined as delivery before 37 completed weeks of gestation. Very preterm infants (born before 32 weeks) and extremely preterm infants (born before 28 weeks) are at higher risk of mortality and morbidity. Advances in perinatal care have improved outcomes for preterm infants, but the boundary for defining preterm birth varies internationally. In the UK, the boundary was lowered from 28 to 24 weeks in 1992. Gestational age and birth weight are related but not interchangeable. Low birth weight (<2500 g) is common in preterm infants, but term infants can also be low birth weight. Preterm infants may be small for gestational age due to intrauterine growth restriction, which is associated with perinatal problems such as perinatal death, fetal distress, meconium aspiration, hypoglycaemia, polycythaemia, and hypothermia. The incidence of preterm birth in developed countries is about 5-7% of live births, with higher rates in the US. Factors contributing to increased incidence include multiple pregnancies, assisted reproduction techniques, and changes in clinical practice. In developing countries, low birth weight is often due to intrauterine growth restriction caused by maternal undernutrition and infection. Improved care in developed countries has not reduced neonatal morbidity and mortality in developing countries where basic care is lacking. Spontaneous preterm labour and rupture of membranes are the most common causes of preterm birth. Smoking is a risk factor, and antenatal smoking cessation programs can reduce preterm birth rates. Multiple pregnancies increase the risk of preterm birth, and assisted reproduction techniques have contributed to this trend. Maternal and fetal complications, such as hypertensive disorders and severe intrauterine growth restriction, are also causes of preterm birth. Outcomes for preterm infants improve with increasing gestational age, but survival and neurological outcomes vary. Infants born before 32 weeks are at higher risk. Preterm infants of multiple pregnancies may have better outcomes than singleton pregnancies of the same gestation. However, outcomes remain poor for infants born before 26 weeks. Preterm birth rates vary by ethnicity, with higher rates in black women. The outcomes for preterm infants have improved in developed countries, but further research is needed to understand the causes and reduce incidence.Preterm birth is a major challenge in perinatal health care. It is a leading cause of perinatal death and is associated with neurological impairment and disability. Preterm infants require prolonged hospitalization and have significant cost implications for health services. Preterm birth is defined as delivery before 37 completed weeks of gestation. Very preterm infants (born before 32 weeks) and extremely preterm infants (born before 28 weeks) are at higher risk of mortality and morbidity. Advances in perinatal care have improved outcomes for preterm infants, but the boundary for defining preterm birth varies internationally. In the UK, the boundary was lowered from 28 to 24 weeks in 1992. Gestational age and birth weight are related but not interchangeable. Low birth weight (<2500 g) is common in preterm infants, but term infants can also be low birth weight. Preterm infants may be small for gestational age due to intrauterine growth restriction, which is associated with perinatal problems such as perinatal death, fetal distress, meconium aspiration, hypoglycaemia, polycythaemia, and hypothermia. The incidence of preterm birth in developed countries is about 5-7% of live births, with higher rates in the US. Factors contributing to increased incidence include multiple pregnancies, assisted reproduction techniques, and changes in clinical practice. In developing countries, low birth weight is often due to intrauterine growth restriction caused by maternal undernutrition and infection. Improved care in developed countries has not reduced neonatal morbidity and mortality in developing countries where basic care is lacking. Spontaneous preterm labour and rupture of membranes are the most common causes of preterm birth. Smoking is a risk factor, and antenatal smoking cessation programs can reduce preterm birth rates. Multiple pregnancies increase the risk of preterm birth, and assisted reproduction techniques have contributed to this trend. Maternal and fetal complications, such as hypertensive disorders and severe intrauterine growth restriction, are also causes of preterm birth. Outcomes for preterm infants improve with increasing gestational age, but survival and neurological outcomes vary. Infants born before 32 weeks are at higher risk. Preterm infants of multiple pregnancies may have better outcomes than singleton pregnancies of the same gestation. However, outcomes remain poor for infants born before 26 weeks. Preterm birth rates vary by ethnicity, with higher rates in black women. The outcomes for preterm infants have improved in developed countries, but further research is needed to understand the causes and reduce incidence.