Q. When does the soul enter the body?
A. Maimonides offers a comprehensive analysis of the soul in his “Eight Chapters” (an introduction to his commentary on the Ethics of the Fathers). For him it seems that “soul” means what we would call personality, which includes but is not limited to one’s spiritual spark.
The question of when the soul, however defined, enters the body, was discussed long before his time in Midrashic and Talmudic sources.
Major views are that the soul enters the “body” at conception, or when the foetus is formed (Sanh. 91b, cf. Men. 97b). Maimonides and others say that what the foetus has is a rudimentary form of soul which matures and reaches higher levels later. The sages believed that even before birth, the soul possesses knowledge, so the learning that comes as the child develops is a rediscovery (Nidd. 30b).
There is a well known view that some aspects of one’s life are decided before birth, especially whom one will marry, but this is more a matter of Divine determination than personal choice.