The gemara (San 91b) quotes a debate that took place between Rebbi and Antoninus as to when the yetzer ha'ra enters a child -- at conception or at the moment of birth:
ואמר ליה אנטונינוס לרבי מאימתי יצה"ר שולט באדם משעת יצירה או משעת יציאה א"ל משעת יצירה א"ל א"כ בועט במעי אמו ויוצא אלא משעת יציאה אמר רבי דבר זה למדני אנטונינוס ומקרא מסייעו שנאמר (בראשית ד, ז) לפתח חטאת רובץ
Rebbi conceded that it was at the moment of birth and not earlier.
We read in our parsha that Rivka was disturbed by the kicking she felt in her womb:
וַיִּתְרֹֽצְצ֤וּ הַבָּנִים֙ בְּקִרְבָּ֔הּ וַתֹּ֣אמֶר אִם־כֵּ֔ן לָ֥מָּה זֶּ֖ה אָנֹ֑כִי
Rashi quotes from the Midrash:
עוברת על פתחי תורה שם ועבר ויעקב רץ ומפרכס לצאת, עוברת על פתח עבודה זרה ועשו מפרכס לצאת.
If the yetzer ha'ra only enters the child just before birth, why was Eisav kicking when he passed a place of avodah zarah in utero? How could he already have had a desire to worship avodah zarah when he had no yetzer ha'ra yet?
I saw this question raised by R' Moshe Yechezkel Tzalach, a sefardic acharon, in his sefer Torat Moshe. He collects a number of different answers, but I would like to suggest an approach that is not on his extensive list.
Rashi comments on the opening pasuk of our parsha, וְאֵלֶּה תּוֹלְדֹת יִצְחָק בֶּן אַבְרָהָם אַבְרָהָם הוֹלִיד אֶת יִצְחָק, that the Torah stresses that Yitzchak was born to Avraham because there were scoffers who tried to deny it:
לפי שהיו ליצני הדור אומרים: מאבימלך נתעברה שרה, שהרי כמה שנים שהתה עם אברהם ולא נתעברה הימנו. מה עשה הקב״ה, צר קלסתר פניו דומה לאברהם, כדי שיאמרו הכל: אברהם הוליד את יצחק. וזהו שכתוב כאן: יצחק בן אברהם היה, שהרי עדות יש שאברהם הוליד את יצחק
It seems incredible that anyone would take these scoffers seriously. We know that it was Sarah, not Avraham, who was infertile. Avraham had fathered Yishmael through Hagar. Secondly, the fact that Sarah could conceive in such old age was a miracle. Does it make sense that G-d would make a such miracle for a rasha like Avimelech and not Avraham?
On the one hand, maybe there is no rhyme or reason sense to the claims of the scoffers. People make all kinds of outlandish claims (if you don't believe me just spend 5 minutes on social media or listen to a Democrat or member of the MSM.) On the other hand, the Torah sees fit to respond to those claims, G-d made a miracle and transformed Yitzchak's visage (see Sifsei Chachamim) in order to refute then. Why bother to refute mishugas if it's just nonsense?
Although the claim that Avimelech physically fathered Yitzchak is too incredible to take seriously, what the Torah might be responding to is the claim that Avimelech was the *spiritual* father behind Yitzchak's birth.
When Yaakov returns from Lavan's home, he sends a message to his brother Eisav telling him עִם־לָבָ֣ן גַּ֔רְתִּי. Rashi comments: בָר אַחֵר גַּרְתִּי בְּגִימַטְרִיָּא תרי"ג, כְּלוֹמַר, עִם לָבָן גַּרְתִּי וְתַרְיַ"ג מִצְוֹת שָׁמַרְתִּי וְלֹא לָמַדְתִּי מִמַּעֲשָׂיו הָרָעִים. If Yaakov kept all 613 mitzvos, does it not go without saying that he did not learn from Lavan's ways?
Sometimes it is hard to get up for davening, especially in the winter when it is still dark and cold and you need to head out early before work. When I walk down my street on those early mornings, I pass a gym at the end of the block. Without fail, even if it is 20 degrees, pitch back and only 6-something in the morning, every parking spot on the block is taken up by women who want to get to their gym class before they start the day. I've said to my wife many times that these women are my inspiration. If they can get up for the gym, then I can get to minyan.
Of course Yaakov did not follow Lavan's evil ways, but he still might still have learned from him and been inspired by him. Like those attending the gym class, Lavan was out of bed first thing in the morning. He was motivated! He was just motivated for all the wrong things. He used his zerizus, his intensity, his chochma, to pursue kinah, taavah, and kavod. Yaakov might have said, "If he can do it, I can do it," and Lavan's behavior might have been his model for good.
Sounds like a good strategy, but remember the end of Rashi: וְלֹא לָמַדְתִּי מִמַּעֲשָׂיו הָרָעִים. The talmidei haBesh"T taught the following yesod: if tzadik is inspired by the deeds of a rasha, that provides a fig leaf, so to speak, for the rasha. Even if the rasha never intended it, since the rasha become an example for good, they earn some degree of redemption. Now, I'm not a tzadik, and the gym goers are not really reshaim, so I take a lesson from them, but Yaakov Avinu was a tzadik, and so he did not want to take a lesson from Lavan. He did not want to give Lavan's wickedness any cover, any positive quality. Therefore, he said וְלֹא לָמַדְתִּי מִמַּעֲשָׂיו הָרָעִים.
Rashi on our parsha writes (25:20) about Rivka: בת בתואל מפדן ארם אחות לבן – להגיד שבחה: שהיתה בת רשע, ואחות רשע, ומקומה אנשי רשע, ולא למדה ממעשיהם. Of course Rivka did not learn to do evil from Besuel, Lavan, and her surroundings. The chiddush is that she did not take her inspiration from them for good either. Her motivation came from inside, independently from what she saw as their bad example, and so in no way can they take credit for serving as a positive, motivating force.
What the scoffers were saying about Sarah is is that the years of chessed with Avraham were not enough to earn Sarah the right to conceive. It was only once she was in Avimelech's home, when she had the negative example of his behavior to inspire her avodah, that she was finally able to merit having a child. (For those who are not happy with seeing this from the Besh"T, see my son's post for a similar vort from the Beis haLevi and other answers as well.) This is what the Torah comes to refute and tell us that it all came from Avraham.
Getting back to our original question about Eisav -- עוברת על פתח עבודה זרה ועשו מפרכס לצאת -- where does it say that Eisav wanted to get out to **worship** avodah zarah? I don't want to give Eisav too much credit, but maybe at this point in his life, in utero, pre-the entrance of the yetzer ha'ra, that was not Eisav's intention at all. The difference between Yaakov and Eisav was where to draw inspiration from. Yaakov wanted to run out to the beis medrash and derive his inspiration from there. Eisav wanted to see what was going on in the beis avodah zarah because the zerizus for evil is sometimes even stronger than that for good, and can therefore serve as a better example for what we should be doing in a positive sense.
Rivka, who knew not to draw any inspiration from Besuel and Lavan's actions, thought this a very dangerous road to go down and was therefore troubled by Eisav's behavior. Inspiration needs to come from inside, not from what we see out in the world.