There's really very little information on that matter. I believe (this is a personal theory that may only work in my head) that the astrological scenario for a given point in time and space binds the soul to the body. The soul that best synchronizes with this energetic scenario will be born in the most appropriate body at that moment.
This would explain why the strongest souls are born with a very long time between each life, as the scenario needs to be ideal (such as major, maybe rare aspects/configurations between specific planets). It could also explain cycles of events and people's involvement with them.
However, I also do believe that with a strong enough soul or with the aid of the Gods, one can choose when to be born and this changes things significantly. Maybe, only maybe, premature births sometimes happen because the soul is forcing birth.
Other than that, I really don't know what could be said about this. I also have never seen any indications of early birth in a chart nor any author that speaks on this subject. At least not consistently.
--- In , <bunpuppet@... wrote:
SPEAKING of which
What in a chart indicates an early birth?
And how does a soul attach to a body exactly after birth? And how does the soul find the body to begin with?