The beginning of physical existence. Spirit on the other hand has always existed. However, a unique individuated form (soul) starts its association with matter at the moment of conception. The personality that is expressed is formed at the moment of birth when the neurons of the brain are formed. The spirit cannot make existence known on Earth except through personality. Personality is the bodily form through which individuality is expressed.
No matter what body is provided, it is the spirit that determines its state. The type of body is subject to the twin law of compensation and retribution. Some souls choose to come into this world because it desires to help or knows it has work to do. Some have no desire, but return because of a Karmic condition to fulfill.
In this world, some children are born into a slum environment, while others grow up surrounded by riches. It is the way a soul expresses itself, whether in high or low estate, though the opportunity of service that it finds itself and its own divinity. All of the great masters of this world were born of low estate. The more difficulties that the soul has to struggle against, the greater the soul can become.
Family planning continues in this world because it is a responsibility for some. However, if a child is to be born, it will be born despite all family planning. We have free will and a conscience to determine the activities surrounding birth control. It always depends on the motive. Those who seek to use birth control purely as a means to enjoy the lust of their body, in order to prevent the consequences, reflect a motive of lowly selfishness (this is in contrast to the pleasures that can be obtained through tantric sex).