Over the past 20 years, the prevalence of premature birth has risen worldwide. In high-income countries, such as Australia, Canada, Spain, and the UK, prevalence is about 4-8%, whereas South America and Asia have prevalences of 15%, increasing to 20-25% in rural areas. In the USA, 12% of births are preterm partly because of the population of African-American women, in whom premature births are 3-4 times more common than in white women. Increasing maternal age, increasing number of multiple pregnancies made possible by assisted-reproduction techniques, and maternal stress could partly explain the worldwide upturn.
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