What is the one chemical (or family of chemicals) that would cause a pregnant mother's body to decide to "jettison its cargo"? The easiest and simplest idea that came to my mind was digestive juices. Certainly, one of the worst things that could happen is that the baby begins to digest the uterus, the placenta and/or itself, and enough digestive juices would also indicate that the baby is matured enough to feed (ie, be born).
Maybe not the digestive system's main digestive juices (ie, stomach acid, etc), but maybe something as simple as the digestive chemicals (amylase/ptyalin and lipase) in the baby's saliva. Babies in the womb seem to float around with their mouths open, and I understand that babies even "breath" the amniotic fluid in and out.
I also understand that the uterus "replaces" the amniotic fluid every four hours (actually a continuous process), so the mother's body may begin the birth process when it starts processing amniotic fluids containing a high enough concentration of digestive chemicals.
What are some other ideas (your pet ideas) about what might trigger birth?