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The true incidence of preterm delivery and preterm labour can only be ascertained if a consistent definition is used, and if the data are population based. The reported incidence of preterm delivery is affected by the method of gestational age assessment, and by the differing definitions of viability used and therefore the registration of every preterm delivery. Further problems occur in the measurement of outcome because of the heterogeneity of preterm birth – delivery may occur near to the 37-week upper limit of gestation where there may be no pathological cause and the baby has relatively few if any problems, or it may occur at the extreme of prematurity at around 24 weeks' gestation, where survival rates are poor, and the risk of severe morbidity in those survivors is high. The birth may be spontaneous or elective (iatrogenic); the spontaneous delivery may be uncomplicated (and the outcome usually better (Chng 1981)) or complicated, for example by prelabour rupture of the membranes. The outcomes of such wide variations in aetiology and gestational age will obviously be dissimilar, and so comparisons are difficult and often of little clinical relevance.
Although there is widespread agreement that ‘preterm’ should refer to a gestational age below 37 completed weeks, there is poor agreement on the definition of the lower limit that defines fetal viability, and on the subdivision of the preterm period into intervals defined by outcome.
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