Nazir, Chapter 2, Mishnah 9. This Mishnah deals with a man who accepted two terms of nezirus upon himself - one for himself and one for when a son will be born to him - and with the question of the order in which he counts them, which is determined by the order in which he mentioned them in his vow.
The first case - he mentioned his own nezirus first:
One who says "Hareini nazir ukesheyihyeh li ben" - "I am hereby a nazir, and when I will have a son" - has accepted upon himself an unspecified nezirus, which is thirty days, and in addition to it a further nezirus for when a son will be born to him. Since he mentioned his own nezirus first, "hischil moneh es shelo" - he begins by counting the general nezirus that he accepted, the one that is not dependent on the birth of the son.
"V'achar kach nolad lo ben" - "and afterward a son was born to him" - suppose that at the end of fifteen days of his nezirus his wife gives birth to a son, so that the birth occurs in the middle of the nezirus he had begun. At that point, "mashlim es shelo v'achar kach moneh es shel beno" - since his own nezirus came first in the vow, he first completes his thirty days of nezirus, shaves his head and brings his offerings, and only afterward begins to count the nezirus of his son.
The second case - he mentioned the nezirus of the son first:
One who says "Hareini nazir kesheyihyeh li ben v'nazir" - "I am hereby a nazir when I will have a son, and a nazir" - the first vow that came out of his mouth is a nezirus for when a son will be born to him, and after it he accepted upon himself an additional nezirus for himself, and both are terms of nezirus of thirty days. Even though the nezirus of the son was mentioned first, at the time of the vow he still has no son - perhaps his wife has not yet conceived, and perhaps she is pregnant and about to give birth - and therefore "hischil moneh es shelo": he begins specifically with the nezirus that he mentioned second, his own nezirus, and waits for the birth of the son.
"V'achar kach nolad lo ben" - "and afterward a son was born to him" - before he completed his thirty days of nezirus, his wife gives birth to a son. Since the nezirus of the son came first in the vow, "maniach es shelo umoneh es shel beno" - he must set aside the nezirus he had begun and count the nezirus of his son, which is the first vow he made, and only afterward "v'achar kach mashlim es shelo" - he completes his own nezirus.
When does he shave and bring his offerings?
He does not shave and bring offerings at the conclusion of his son's nezirus, since he is still standing in the middle of the nezirus that he had begun. He therefore waits until he completes his original nezirus, the one he mentioned second: he was fifteen days into his nezirus, his son was born, he counted thirty days for the son's nezirus, he completed the remaining fifteen days, and only then does he shave and bring two sets of offerings.
The reason for this: if he were to come and shave for his son's nezirus, only fifteen days would remain for him, and there is no shaving unless there has been hair growth of at least thirty days.
When his own nezirus is longer than thirty days:
The matter is different when the son's nezirus is thirty days while the nezirus he had begun was, for example, one hundred days. If after fifteen days his wife gives birth and he counts the son's nezirus, he may shave at the end of the thirty days of his son's nezirus, since he still has eighty five days remaining, in which there will be hair growth of thirty days. It turns out that he shaves first for his son's nezirus, and afterward shaves for the nezirus that he had begun at the outset.
The opinion of the Rambam:
The Rambam holds that even in a case where both terms of nezirus are of thirty days - his son's nezirus thirty days and his own nezirus thirty days - he shaves and brings offerings at the conclusion of his son's nezirus. For if after fifteen days of his nezirus his son was born and he counted the thirty days of the son's nezirus, then even though only fifteen days remain for him, he extends his nezirus until there is hair growth of thirty days from the conclusion of his son's nezirus, and then he shaves.
In summary: In this Mishnah we learned that the order in which the two terms of nezirus are counted is determined by the order in which they were mentioned in the vow: if he mentioned his own nezirus first, he completes it and afterward counts his son's; if he mentioned the son's nezirus first, he sets aside the nezirus he had begun, counts his son's, and afterward completes his own. We also examined the question of when the shaving and the offerings take place, which depends on hair growth of thirty days, and the opinion of the Rambam that even with two terms of nezirus of thirty days he shaves at the conclusion of his son's nezirus by extending his own nezirus.