Even if birth is "a long process"
the defining moment is the first breath
However, the moment of the first breath is often missed
I think that is the point. The 'first breath' is open to interpretation. It is not that cut and dry. Some take their 'first breath' while they are still partially in the mothers body. Does that count? Or do we wait until they are fully freed and on their own?
The difference in time between various interpretations may mean bigger changes in the house cusps than some of the house systems do.