Key facts preterm birth

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 [1]


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 situation

  • Worldwide, estimated 13.4 million babies are born preterm annually [2] – 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 [3].
  • 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 [4].
  • Across 184 countries, the rate of preterm birth ranges from 5% to 18% of babies born [4].
  • 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 [4].
  • 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 [5].


Causes & risk factors

Preterm birth occurs for a variety of reasons [6,7]. The majority of preterm births happen spontaneously. Experts have identified several factors which may increase the risk of preterm birth.

Some risk factors are “modifiable,” meaning they can be changed to help reduce the risk, while others cannot be changed. However, in about 50% of cases no cause for the baby born too early can be identified.

Medical conditions

Pregnancy historyFor example a mother having born a preterm baby before is at higher risk for another preterm birth within the next pregnancy.
Multiple pregnancyThe most common complication of a multiple pregnancy is preterm birth.
Assisted reproductive technologiesReproductive treatments are linked to multiple pregnancies and hence to preterm birth.
Uterine or cervical abnormalitiesFor example, a short cervix might favour preterm birth: during pregnancy, the cervix gradually softens, decreases in length as the baby grows to prepare birth. As consequence the cervix might begin to open too soon.
Infections and chronic conditionsInfections of the uterus (e.g. through urinary tract infections or bacterial vaginosis , an imbalance of the bacteria that are normally present in the vagina) or non-communicable diseases such as for example diabetes, high blood pressure, asthma or renal and cardiac disorders.
Pregnancy complicationsGestational diabetes and pre-eclampsia are two examples of typical pregnancy complications. Due to preeclampsia women may need to give birth early to avoid serious health problems for both, the mother and the baby.
Genetic influenceSpecific foetal and maternal genotypes (a person’s type of genes for a particular inheritable trait) are associated with the risk of preterm delivery.

Modifiable lifestyle risk factors

Lifestyle patternsFactors like an unbalanced diet, underweight or overweight (obesity), smoking, alcohol or drug consumption and a high stress level.
HealthcareNo or late antenatal care might inhibit identification and therapy measures at an early stage.
EnvironmentExposure to environmental pollutants (e.g., air pollution and environmental chemicals) but also risk factors associated with climate change such as increasing temperature and heatwaves [7].

Demographic factors

AgeParticularly young (< 17) or older women (> 35) are at higher risk for preterm birth.
Sociodemographic determinantsSocioeconomic disadvantages (low education level, low income, few social support) might play a role for preterm birth.
EthnicallyThere are higher preterm birth rates reported in black than white women [8, US data].


  1. World Health Organization. Preterm birth. 2016. Available from: (Date of visit: 05.12.2016)
  2. World Health Organization. Born too soon: decade of action on preterm birth. 2023. Available from:
  3. 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.
  4. World Health Organization. WHO fact sheet on preterm birth. Available from: (Date of visit: 18.04.2017)
  5. 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:  (Date of visit: 29.11.2018)
  6. Goldenberg R, Culhane J, Iams JD, Romero R. Epidemiology and causes of preterm birth. 2008. The Lancet. 371(9606): 75–84.
  7. Chersich M, Duc Pham M, Areal A et al. Associations between high temperatures in pregnancy and risk of preterm birth, low birth weight, and stillbirths: systematic review and meta-analysis. 2020. BMJ;371:m3811 doi:
  8. Keller M, Saugstad O, van Steenbrugge G, Mader S, Thiele N. European Foundation for the Care of Newborn Infants. Caring for Tomorrow. EFCNI White Paper on Maternal and Newborn Health and Aftercare Services. Munich 2011.