The involvement of progesterone in the onset of human labour.

Progesterone is essential for the maintenance of pregnancy, and acts to promote uterine quiescence. In many species progesterone levels fall prior to the onset of labour but no fall has been observed in humans. The concept of 'functional' progesterone withdrawal in association with labour has been proposed and a number of mechanisms have been suggested. Recent potential mechanisms of functional withdrawal include changes in progesterone receptor (PR) isoforms, regulation of PR responsive genes through nuclear factor kappaB (NF-kappaB) promoter sites and the non-genomic effects of progesterone or its metabolites.