Amniotes evolved the ability to lay eggs on dry land.This method of egg laying required internal fertilisation and the retention of the developing egg within the reproductive tract while the shell was created.To retain the egg within the reproductive tract the regular contractions of the smooth muscle of the reproductive tract had to be arrested.It seems likely that progesterone has been used to signal arrest of contractions of the reproductive tract since the evolution of the amniotes.Withdrawal of progesterone then allows resumption of reproductive tract contractions and the laying of the egg.This process is likely fundamental to pregnancy in mammals which are also amniotes.For the majority of pregnancy in mammals the contractions of the uterus must be suppressed, the with then to allow delivery of the fetus the contractions must be allowed to occur.In most mammals the onset of labour is regulated by an abrupt fall in circulating progesterone concentrations or a rise in circulating oestrogens.In humans, however, no dramatic change is observed in circulating steroid concentrations prior to labour and the mechanisms regulating the onset of labour have remained obscure.In a series of studies we have shown that placental production of the peptide hormone CRH increases exponentially across gestation peaking at the time of labour.