Neuroendocrinal mechanism of parturition
These hormones are progesterone and estrogens. They are actually secreted by the placenta, in increasing amounts as the fetus matures:



(Colorado State, R. Bowen)

You can see that the placenta produces both types of hormone in increasing amounts until birth. Additionally, the hormone relaxin is secreted by the placenta and thought to aid in parturation:


  Relaxin is a hormone thought to act synergistically with progesterone to maintain pregnancy. It also causes relaxation of pelvic ligaments at the end of gestation and may therefore aid in parturation. In some of the species in which relaxin is known to be produced, it is produced by the placenta, while in others, the major source is the corpus luteum. In some species, relaxin is produced by both the corpus luteum and placenta. - R. Bowen