Neuroendocrinal mechanism of parturition

Question

My book reads,

"The fetus signals that it is mature by secreting certain hormones that diffuse across the placenta into mother's blood and cause the secretion of oxytocin from her posterior pituitary."

I would like to know what exactly are these hormones and their sites of production in the foetus, as well as their mode of action.

Also, I was wondering whether these hormones are released even during premature births? I doubt this because what I understand from the lines of my book is that it has to be a signal by the foetus that it is 'fully developed', whereas if it is not secreted it would not stimulate the secretion of oxytocin. This is creating a bit of confusion.

A detailed explanation would be a great help

Thank you

Answer 1

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