(21) Isaac pleaded with GOD on behalf of his wife, because she was infertile; and GOD responded to his plea, and his wife Rebekah conceived. (22) But the children struggled in her womb, and she said, “If so, why do I exist?” She went to inquire of GOD,

(2) ותאמר אם כן means AND SHE SAID, “IF the pain of pregnancy be so great, (3) למה זה אנכי WHY IS IT that I longed and prayed to become pregnant?” (Genesis Rabbah 63:6).

...Rebekah asked women who had children if they had had a similar experience. They responded that they did not. (2) Rebekah then said, if it, namely, the manner and custom of pregnancy, be so as they said, wherefore do I undergo such an abnormal pregnancy.

(1) AND SHE SAID: IF IT BE SO, ‘LAMAH ZEH ANOCHI?’”If the pain of pregnancy is so great, lamah zeh anochi (why did I) pray for and aspire to pregnancy?” Thus Rashi. But it is not correct. Rabbi Abraham ibn Ezra says that she asked the women if they had experienced such pains, and they said, “No,” whereupon she said, “If the matter and custom of pregnancy be as they said, lamah zeh anochi, why am I beset with an unusual pregnancy?” Now according to this exposition, the verse is missing and not complete.
The correct interpretation in my opinion is that she said, “If it shall be so with me, lamah zeh anochi, [why am I in the world]? Would that I did not exist, that I should die or never have come into existence.” This is similar to the verse, I should have been as though I had not been.

(1) ותאמר אם כן, after it is a fact that these fetuses already are at each other’s throats I have reason to be afraid that one of them will die so that I endanger myself by giving birth to them, causing one of them to be stillborn. (2) למה זה אנכי, why were my relatives so concerned that it would be I who provide the seed for Yitzchok when they said את היי לאלפי רבבה, “may it be you who will be the source of millions.” (24,60) Also, why did my husband insist that I become the mother of his children?

(2) אם כן, “if there is so much pain involved in pregnancy, why do I have to remain alive?” I prefer to die immediately and avoid suffering the pains of pregnancy and birth.

(1)“And the children struggled within her, etc.” This struggling occurred when she would pass by the entrance of the study hall of Shem and Ever — Jacob would struggle to emerge while Esau would hold him back. And when she would pass by the entrance of a place of idol worship, Esau would struggle to emerge while Jacob would hold him back. She thought that this couldn’t be the case, rather that there was only one child in her womb who wanted to emerge both at the entrances of study halls and at the entrance of places of idol worship. If so, Heaven forbid, perhaps there are two [divine] authorities. Therefore, she said Why is this happening to me? — meaning, I am like other women who worship idols, so what advantage do I have over them if, Heaven forbid, there are two authorities? Therefore, she went to inquire of the Lord — meaning, to inquire about the true existence of God, what it really is.