If you really wanted to look for a potential factor it would be cosmic particles. They are passing right through us all day and all night, passing through the latticework of atoms like tennis balls thrown through scaffolding. They do, very occasionally, hit the structure.

The trouble with linking cosmic particles to astrology is the fact that we are indeed bombarded with them at all times, and that makes a nonsense of the importance attached by astrology to the exact time and place of birth.

It was obvious for some weeks before he was born that my son was a considerably more laid-back baby than his older sister. That there is some sort of cosmic significance to the moment of birth-by that point in development not much more than a (traumatic) change in environment-doesn't make logical sense. How could cosmic particles or forces acting across vast distances be stopped dead by a few centimetres of flesh and amniotic fluid?

Birth time isn't calculably related to the moment of conception with any great accuracy-there's sufficient variation in gestation period that only about 5% of babies are born on their due date.  On a cosmic scale, even within the scale of an individual life, there's a four-week period within which the moment of birth is as near arbitrary as makes no difference.

Unless we postulate that the time of birth is influenced by the guiding stars.  But my own birth was induced early when my mother developed pre-eclampsia.  It altered my star sign, but a generation or so earlier we would probably both have died.  For cosmic forces to account for relatively new medical technology implies a level of predestination rather incompatible with any notion of free will.