The mechanism of human labor is a complex issue. A series of partly still unknown processes of hormonal, neural, biochemical and biophysical nature lead to the dramatic changes of uterine physiology at the end of pregnancy. The uterus can be separated in two parts: the corpus, mainly containing myometrium, and the cervix consisting of connective tissue. During pregnancy the myometrium is quiescent and the cervix firm and closed, prerequisites for an undisturbed growth of the embryo, During labor the myometrium has to contract regularly while the cervix needs to lose its strength allowing the passage of the fetus through the lower uterine segment. This change in the roles of the corpus and cervix is a gradual process throughout the last part of pregnancy until the onset of spontaneous labor. Many factors have been thought to be involved, however there is as yet no final theory to explain the onset of human labor (Fig. 1) [1,2]. One of the candidates as a trigger hormone for the initiation of labor is oxytocin. The concentration of oxytocin in the maternal plasma gradually rises throughout pregnancy [3]. However there is no dramatic rise before the onset of labor. This has continuously been used as an argument against the involvement of oxytocin as a trigger for human labor. It is however well known from clinical work that apart from the concentration of a hormone in the peripheral circulation, another important parameter of its action is the sensitivity of the end organ. We have therefore conducted studies to measure oxytocin sensitivity during the last days before the onset of spontaneous labor, while measuring oxytocin plasma levels at the same time [4]. Fig. 2 shows that there is a clear rise in uterine sensitivity to oxytocin during the last few days