The practical answer is because it's much easier to determine (unless you have a very uneventful and well documented sex life...)
The metaphysical answer - because you don't have a fully formed 'person' until the moment of birth (when the cord is cut)
The scientific answer - the result would be the same (the cosmic 'clock' would read the same 'time') only offset by 9 months. In other words if it's 2pm in New York (conception) then we know it's 9pm in London (birth) and from either position can calculate that it's midnight in Moscow (the astrological result)
This is a question relevant to the metaphysical school of thought, where the planets are thought to influence the person at the time of birth. The answer is no! Not because of any magical properties of the womb, but because the baby isn't a fully formed person until separated from the mother.
Why doesn't the presence of those delivering a baby affect the persons birth chart? This is a question which assumes that gravity is the mechanism by which a planet affects astrology and birth. If gravity were responsible, then yes that midwife probably has ten times more gravitational pull (which is highly sensitive to vast distances) than Saturn does. Fortunately, gravity isnt a factor, so it's quite safe to use a plump midwife. "ah, i see you have Mrs. Holbrook in your fourth house..."
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