Trigger for Onset of Birth

Unlike the case in ruminants like sheep, the precise endocrine trigger for onset of the birth process (progesterone withdrawl) is unknown in human or great apes. Humans appear to be unique in that there is no fall in circulating progesterone levels at the time of birth although blockade of progesterone using the antagonist RU486 can initiate parturition at any time during pregnancy (Smith 2007). The timing of birth in humans may be linked to the development of the placenta and to placental expression of the gene for cortocotropin releasing hormone (CRH) (Smith 2007). Maternal circulating levels of CRH rise exponentially during pregnancy and peak at the time of delivery (Smith, 2007), this pattern is absent in other primates studied (i.e. baboons and macaques, Challis et al., 2000). While gestation period in chimpanzees shows only a 10 – 12 day variation, it is difficult to predict with any accuracy the date of delivery for human infants (normal term gestation has a range of 37 to 42 weeks), raising the question of whether the triggering mechanism is different.