Preterm birth is defined by the World Health Organisation as birth occurring prior to 37 weeks gestation (normal gestation is 40 weeks) and is the cause of the majority of perinatal morbidity or mortality. By this definition the incidence of preterm birth is 5-10% in the western world . Predicting preterm delivery is extremely difficult, though there are known risk factors that
Abbreviations: cAMP, cyclic 3',5' adenosine monophosphate; ELISA, enzyme-linked immunosorbent assay; FLIPR, fluorescence imaging plate reader; OT, oxytocin receptor; V1a, vasopressin V1a receptor; V1b, vasopressin V1b receptor; V2, vasopressin V2 receptor; cys, cysteine; pro, proline; tyr, tyrosine; ile, isoleucine; thr, threonine; orn, ornithine; gly, glycine; phe, phenylalanine; asp, aspartate; asn, asparagine; trp, tryptophan; arg, arginine; p.o., oral; i.v., intravenous; CHO, Chinese hamster ovary; HEK, Human embryonic kidney; pmp, pentamethylenepropionic acid; tic, tetrahydroisoquinoline-3-carboxylic acid; 7TM, seven transmembrane; GPCR, G-protein coupled receptor; PSA, polar surface area. IC50, Ki, pKi, AD50, ID50, pA2, pKB and DR10 are defined in the section 'Methods used to measure oxytocin antagonist potency'.
significantly increase the risk of a preterm delivery. These include multiple pregnancy, previous preterm delivery, vaginal infection, low socio-economic class, intravenous drug use and multiple sexual partners. Recently a diagnostic/ prognostic test using foetal fibronectin has become available, though sensitivity and specificity is not high enough to provide a conclusive diagnosis .
Clinical outcome is associated with the degree of prematurity. Those infants born before 32 weeks gestation (about 25% of the total preterm births) have a significantly higher risk of death or disability. The majority of infants born at 32 weeks have some clinical morbidity . Mortality drops from about 80% at 23 weeks to less than 10% at 30 weeks gestation. Long-term disability can range from severe (such as cerebral palsy or blindness) to milder affects such as below-average school achievement or mild respiratory problems.
The acute financial costs to a hospital are also dependent on the gesta-tional age of the infant. The average cost of a term birth was $1,100 in a US hospital in 2003 . At 36 weeks gestation, the cost was $2,600 per infant which rose to $203,000 at 25 weeks gestation. The long-term costs for care of those that have been disabled by preterm labour have not been estimated, but will almost certainly be significantly higher than the acute costs. For example, the lifetime costs of caring for an individual with cerebral palsy have been estimated as being close to $1 million .
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