How does the Torah prove that Avimelech was not the father of Yitzchok?
(25,19) “And these are the generations of Yitzchok the son of Avrohom; Avrohom begot Yitzchok.”
Rashi explains that the phrase “these are the generations of Yitzchok” refers to Ya’akov and Eisav who are mentioned later in the parsha, but what problem is Rashi resolving with this explanation?
One explanation is that he is showing how the Torah is countering the rumour that Avimelech was the real father of Yitzchok. Because Chazal taught in the gemora that a mamzer (illegitimate child) cannot have children. Therefore the Torah states “these are the generations of Yitzchok (Ya’akov and Eisav) - Avrohom begot Yitzchok”. That is, from the offspring of Yitzchok it is clear that Avrohom was the father of Yizchok, because if Sarah had become pregnant from Avimelech then Yitzchok would have been a mamzer and thus would not have been able to have children.
Another explanation is according to the teaching that since Yitzchok was a Tzaddik born from a Tzaddik he begot Ya’akov, and since Rivka was a Tzaddekess born from a rasha (a wicked person) she begot Eisav. Therefore, the Torah states “these are the generations of Yitzchok (Ya’akov and Eisav)” from which it is clear that “Avrohom begot Yitzchok”, because if Sarah had become pregnant from Avimelech then Yitzchok would also be a Tzaddik born from a rasha, and so how could he have begot Ya’akov?
Why did Hashem hint to Rivka that Rebbi and Antoninus would be amongst her offspring?
(25,23) “And Hashem said to her: Two nations are in your womb.”
Rashi explains that the two nations allude to Rebbi and the Roman emperor Antoninus, and we can explain why Hashem told her this according to the gemora in Sanhedrin 91b which teaches that Antoninus asked Rebbi from which time does the Evil Inclination rule over man - from the time of the formation of the embryo or from the time it goes out into the world? Rebbi replied that it was the former, but Antoninus retorted that if this was the case then it would rebel in its mother’s womb and go out (before its time), and therefore it must be from the time that it goes out into the world. Rebbi subsequently agreed that this was so, bringing a posuk as a proof.
According to this we can say that this the explanation of the posuk “and the children struggled within her”, that when she passed a place of idol worship Eisav and struggled to go out she thought that it must be that the Evil Inclination comes at the time of the formation of the embryo. Therefore she said “why this am I” - why did I pray to become pregnant? Since the baby might rebel in the womb and go out before its time I will give birth in vain (to an aborted foetus)!
Therefore, “Hashem said to her: Two nations are in your womb” - Antoninus and Rebbi, who hold that the Evil Inclination is given to the child only from the time that it is fitting to go out into the world. Therefore, it is certainly now the time for them to go out and that is why they are struggling to break out.