Definition & epidemiology
The World Health Assembly (= decision-making body of WHO) provided the first definition of preterm birth in 1948. Nowadays this is the most extensively used and accepted definition of preterm birth.
The average pregnancy lasts for approximately 37 to 42 weeks. Every baby born before completion of 37 weeks of pregnancy (also called weeks of gestation) is considered as preterm. The following subcategories are used for further distinction:
- extremely preterm: <28 weeks of gestation
- very preterm: 28 to <32 weeks of gestation
- moderate to late preterm: 32 to <37 weeks of gestation
- late preterm may still be differed with referring to 34-37 weeks of gestation 
Preterm babies are also differentiated in terms of unusually small body length and weight for the number of weeks of pregnancy (gestation period, also called gestational age). Babies born preterm have much higher rates of low birthweight. Low birthweight refers to babies who are born weighing less than 2,500 grams (about 5.51 pounds), very low birthweight to babies with a birth weight less than 1,500 grams (about 3.30 pounds). The concept of small for gestational age describes babies who are smaller than the usual average for the number of weeks of pregnancy.
- Worldwide, estimated 15 million babies are born preterm annually  – that means 1 baby in 10 is born premature. Worldwide. And the number is rising.
- Preterm birth complications are the main cause of global under-5 deaths .
- Preterm birth complications were responsible for nearly 1 million deaths in 2015.
- Three-quarters of them could be saved with current, cost-effective interventions.
- Many survivors face a lifetime of disability, including learning disabilities and visual an hearing problems. .
- Across 184 countries, the rate of preterm birth ranges from 5% to 18% of babies born. .
- Inequalities in survival rates around the world are stark. In low-income settings, half of the babies born at or below 32 weeks die due to a lack of feasible, cost-effective and basic care, e.g. warmth, breastfeeding support, basic care for infections and breathing difficulties. .
- In Europe, preterm birth is one of the two leading causes for neonatal mortality and accounts for more than half of all deaths in later childhood. Prevalence rates of preterm birth range from 5.4 to 12.0 % – an average of 7.3% of all live births .
 World Health Organization. Preterm birth. 2016. Available from: http://www.who.int (Date of visit: 05.12.2016)
 Althabe F, Howson CP, Kinney M, Lawn J, World Health Organization. Born too soon: the global action report on preterm birth. 2012. Available from: www.who.int/pmnch/media/news/2012/201204_borntoosoon-report.pdf
 GBD 2015 Child Mortality Collaborators, and others. Global, regional, national, and selected subnational levels of stillbirths, neonatal, infant, and under-5 mortality, 1980–2015: a systematic analysis for the Global Burden of Disease Study 2015. 2016. The Lancet, 388 (10053): 1725–1774.
 World Health Organization. WHO fact sheet on preterm birth. Available from: http://www.who.int/mediacentre/factsheets/fs363/en/ (Date of visit: 18.04.2017)
 EURO-PERISTAT Project. European Perinatal Health Report. Core indicators of the health and care of pregnant women and babies in Europe in 2015. Available from: http://www.europeristat.com/images/EPHR2015_Euro-Peristat.pdf (Date of visit: 29.11.2018)