When does a soul come into the world?

When does a soul come? A popular guru （南怀瑾）once said:"Many souls are coming by the bed while a couple is making love." He was wrong. Samskara tells those souls are expecting a body where to go. Also, they are not coming by the bed of a pregnant woman is going to deliver a baby. Only the one who is fit for the samskara. And when the life is coming into the seed, that is the time where the soul comes.