At what point is a new soul created according to the Catholic Church?

It seems clear that the Catholic Church rejected Origen's claim that souls were created and existed before conception and birth. Does the Church pinpoint fertilization of the egg as the moment a soul is created? Is it upon implantation, or once the heart starts beating? Is it at some other stage before birth? Maybe it is upon the taking of the first breath, since the Greek word *pneuma* ("breath") is associated with the word "soul". Or does it happen at some other point in time? According to the Catholic Church, when do souls start to exist?