Rashi (in Pasuk 11) ¹ : If he marries the Y'fas Tohar, he will come to hate her, ² and they will bear a son who will become a ben Sorer u'Moreh. ³

Da'as Zekenim: Because he took a Y'fas To'ar, he will have two wives in his house who quarrel, one of whom he will hate. Then one of them will bear him a Ben Sorer u'Moreh. ⁴ And ultimately, the Torah writes "ve'Chi Yih'yeh be'Ish Chet Mishpat Maves"

Kidushin, 68a: It implies that they are from the same nation ¹ - to preclude where one of them is a Nochris.

Kidushin, 68a: It means that the one's marriage is beloved, and the other's hated ¹ - because it is subject to a La'av. ² And the fact that the Torah writes "Ki Sih'yenah" teaches us that Kidushin on Chayvei La'avin takes effect.

Bava Basra, 142b: "Veyaldu" implies that it is only a baby who is born that detracts from the extra portion of a B'chor, but not a fetus, even though he does receive a portion of his deceased father's property when he is born. Consequently, if a man dies leaving two boys and a pregnant wife, one divides his property into three, of which the B'chor receives one part - ignoring the fetus - whereas the remainder is divided into three parts of which each of the three sons (including the fetus when he is born). ¹

Bechoros, 47b: "Veyaldu" implies a regular birth, to preclude a B'chor who is born by ceasarian section, where he is not entitled to a double portion. ²

Kidushin, 68b: From the combination of :"Ki Sih'yena le'Ish" and "Veyaldu lo" we learn that whenever Kidushin takes effect the baby goes after the father - but when it does not - a Nochris who is married to a Yisrael - it goes after the mother. ³

Sifri: "Vayaldu lo" implies that the baby belongs to him - to preclude the baby of a Shifchah and a Nochris and where it is not known as to whether it is a seventh-month child of the woman's second hunsband or the ninth-month baby of her first one, ⁴ all of whom are not subject to the Din of Bechorah.

Sifri: "Banim" teaches us that, even though daughters inherit when there are no sons, they are not subjet to the Din of Bechorah. ⁵

Bava Basra, 127a: "Veyaldu lo Banim" implies that when the baby is born, it is known to be a boy - to preclude a Tumtum who is torn open after he is born and found to be a boy, who does not diminish from the portion of the Bechorah. ⁶

Sifri: To incorporate a Bchor from a woman who is a Chayvei Kerisos bi'Yedei Shamayim ¹ in the Din of B'chor. ²

Bava Basra, 126b: It implies that the baby must be known to be a boy when it is born - to preclude a Tumtum who is torn open only after he is born from the Din of Bechorah.

Sifri: Refer to 21:15:152:1.

Bava Basra, 127b: "ha'Ben", 've'Lo Tuntum ve'Androginos'; "ha'Bechor", 've'Lo Safek' - to preclude from the Din of B'chor a Tumtum, an Androginus and a Safek - as to which of two women (who both gave birth at the same time) ¹ is his mother.

Kol Eliyahu and Divrei Eliyahu: We discuss a man who married a woman, divorced her, and remarried. The latter gave birth after seven months, and afterwards his divorcee gave birth to a nine-month baby. Who is considered the Bechor? 'Hated' hints to a divorcee. The father wants to give Bechorah to the first son born. The Torah teaches that it depends on conception (Reishis Ono). ¹

Abarvenel: Because through Hashgachah P'ratis, ¹ the hated wife will always ² give birth to the B'chor. ³