After 20 years of marriage, Yitzchak's prayers are answered and Rivka conceives twins. The pregnancy is extremely painful. G-d reveals to Rivka that the suffering is a microcosmic prelude to the worldwide conflict that will rage between the two great nations descended from these twins, Rome and Israel. Esav is born, and then Yaakov, holding onto Esav's heel. They grow and Esav becomes a hunter, a man of the physical world, whereas Yaakov sits in the tents of Torah developing his soul. On the day of their grandfather Avraham's funeral, Yaakov is cooking lentils, the traditional mourner's meal. Esav rushes in, ravenous from a hard days hunting, and sells his birthright (and its concomitant spiritual responsibilities) for a bowl of lentils, demonstrating his unworthiness for the position of firstborn. A famine strikes Canaan and Yitzchak thinks of escaping to Egypt, but G-d tells him that because he was bound as a sacrifice, he has become holy and must remain in the Holy Land. He relocates to Gerar in the land of the Philistines, where, to protect Rivka, he has to say she is his sister. The Philistines grow jealous of Yitzchak when he becomes immensely wealthy, and Avimelech the king asks him to leave. Yitzchak re-digs three wells dug by his father, prophetically alluding to the three future Temples. Avimelech, seeing that Yitzchak is blessed by G-d, makes a treaty with him. When Yitzchak senses his end approaching, he summons Esav to give him his blessings. Rivka, acting on a prophetic command that the blessings must go to Yaakov, arranges for Yaakov to impersonate Esav and receive the blessings. When Esav in frustration reveals to his father that Yaakov has bought the birthright, Yitzchak realizes that the birthright has been bestowed correctly on Yaakov and confirms the blessings he has given Yaakov. Esav vows to kill Yaakov, so Rivka sends Yaakov to her brother Lavan where he may find a suitable wife.
The Calculus to Multiply
“The children (Esav and Yaakov) agitated within her (Rivka), and she said, ‘If so, why am I thus?’...” (25:22)
King Chizkiahu refused to procreate because, through prophetic insight, he saw evil people amongst his offspring.
Yishayahu the Prophet criticized him, telling him he had no business entering into the calculations of Heaven: "What the Torah requires from you, that is what you do." (Talmud Bavli, Berachot 11a)
Therefore, if it not for a direct command from G-d, "Be fruitful and multiply," Chizkyahu's reckoning was correct. If you know that amongst your progeny there will be evil offspring, better not to have any at all.
However, this only applies to the male of the species. Women have no Torah commandment to procreate.
“If so, why am I thus?”
With this we can understand Rivka's question.
Rashi tells us that whenever Rivka passed by the doors of a house of idol worship she felt her unborn fetus struggle to emerge. She knew that her child was destined to be an idol worshipper. "Why am I thus? Why should I labor to give birth to a child who will be evil; I am not Yitzchak. I am a woman, and I have no obligation to reproduce.”
Source: Rabbi Yitzchak Zev Soloveichik