Prematurity is the leading cause of perinatal death and disability. Preterm birth before 37+0 weeks of gestation accounted for 7.6% of all live births in England and Wales in 2005. Although preterm birth is defined as delivery before 37+0 weeks of gestation, the majority of prematurity-related adverse outcomes relate to birth before 33+0 weeks of gestation. Mortality increases from about 2% for infants born at 32 weeks of gestation to more than 90% for those born at 23 weeks of gestation. Two-thirds of preterm births are the consequence of spontaneous preterm labour and/or preterm prelabour rupture of membranes (PPROM). The rate of spontaneous preterm birth continues to rise globally despite efforts to the contrary, and interventions aimed at reducing preterm birth have been largely disappointing. Cervical cerclage was first performed in 1902 in women with a history of mid-trimester abortion or spontaneous preterm birth suggestive of cervical ‘incompetence’, with the aim of preventing recurrent loss. Cervical incompetence is an imprecise clinical diagnosis frequently applied to women with such a history where it is assumed that the cervix is weak and unable to remain closed during the pregnancy. However, recent evidence suggests that rather than being a dichotomous variable, cervical ‘competence’ is likely to be a continuum influenced by factors related not solely to the intrinsic structure of the cervix but also to processes driving premature effacement and dilatation. While cerclage may provide a degree of structural support to a ‘weak’ cervix, its role in maintaining the cervical length and the endocervical mucus plug as a mechanical barrier to ascending infection may be more important.
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