Perinatal epidemiologists no longer speak much of "prematurity". Instead the World Health Organization recommends that infants born before 37 completed weeks of gestation should be designated "preterm" and those of birthweight less than 2500 g "low birthweight". Both definitions have obvious drawbacks--the former because menstrual dates are often unknown or unreliable, the latter because it does not separate preterm from small-for-gestational-age infants. Whatever the classification of these small infants there is no dispute about their high mortality and morbidity; in the United Kingdom they account for over two-thirds of first-week deaths.