Nazir 14 - Nazir because of the birth of a son
If one says, "I am becoming a nazir" and then also "When a son is born to me, I will become a nazir" - then he becomes a nazir and begins counting his thirty days. If a son is born to him before he completes his own period of nazir (which he did not expect), then he completes his own, and after that he becomes nazir for the birth of his son.
However, if he says, "I will be a nazir when I have a son born to me, and I am also a nazir now" - then he indeed becomes a nazir now. But when a son is born to him, he sets aside his own time of nazir behavior and starts the one which he promised for the birth of his son. He completes that, and then resumes his own counting of nazir days. This is because he fully expected that his son will be born to him before his own nazir period completes.
A question comes up: at the end of his son's nazir period, he is supposed to shave his head, like he does at the end of every nazir period, but he should not do it - because he is now back to his own nazir - and as such he is not allowed to touch his hair. Here there are two points of view. The Talmud then discussed combinations of various nazir periods, overlapping and taken for various lengths of time.
Art: The Return of the Prodigal Sonby Rembrandt Van Rijn