The authors reviewed the literature in order to try to work out the different mechanisms that are responsible for labour starting. A role seems to be played by the decidua of the uterus, by the placenta and by the fetal membranes as well as the very important role of prostaglandin synthesis. It does seem as though there is no single factor that is important for starting labour. If we knew more about the physiology of human labour it would perhaps be easier to control premature labour.