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Although the definition of preterm birth is birth before 37 completed weeks, the major transition in terms of needing special care occurs between 34 and 37 weeks. The Homo sapiens neonate is born much more immature than other anthropoid species, perhaps because earlier birth has evolved to avoid the large head of the human fetus becoming impacted in the small pelvis of the mother, who has become adapted to a bipedal gait. The main burden of preterm birth exists in developing countries. There are no accurate recent worldwide data, but estimates of preterm birth rates range from 5% in developed countries to 25% in developing countries. The preterm delivery rate has been relatively stable at 5–10% in developed countries for many years. The North Thames database of 517,381 pregnancies demonstrates significant ethnic variation in preterm birth rates, with higher rates in black women. This is associated with an accelerated rate of maturity in the black fetus and neonate, with correspondingly lower gestation-specific neonatal mortality rates below 38 weeks, and higher at 38 weeks of gestation and beyond. Ethnic differences can explain only a very small proportion of global preterm births. The greatest aetiological factor worldwide is infection, mainly due to malaria and HIV. In developed countries, iatrogenic delivery is responsible for almost half of the births between 28 and 35 weeks; hypertension and pre-eclampsia are the major pathologies. Other factors include multiple pregnancy, intrauterine growth restriction, maternal stress and heavy physical work.