The children struggled within her. She said, “If so, why am I thus?” She went to inquire of Hashem. Hashem said to her, “There are two nations are in your womb; two regimes shall be separated from your insides; might will pass from one regime to the other; and the elder shall serve the younger. 
Rivka’s reaction is difficult to understand. Why would a painful pregnancy get her to doubt her existence? What insight did she hope to gain by seeking Divine insight? The forecast of painful pregnancies goes back to Chava! Nothing new or remarkable there. Was there anything so out of the ordinary about the stirrings she felt inside her?
The answers she received don’t seem to address her questions. Explanations about the future roles of her two sons do not tell her anything about how to deal with her pregnancy pains. What is the difference between “two nations” and “two regimes?” The two phrases don’t match each other, either. Since the first speaks of two entities that are within her, the second one should speak of two that exit or leave her – not that “separate” from her. Why did Hashem throw in the information about the older son serving the younger? It may have been important, but had nothing to do with Rivka’s question!
Now, we may realize that a number of these questions are behind the position of Chazal that Rivka in fact was not puzzled by the pain or the stirrings, but about the bizarre behavior of the child (at that point she was aware of only a single fetus) within her. She felt the stirrings when she passed a place of kedushah – but also felt them when near a shrine to avodah zarah. This is where her questions began, but there was more to them than just this confusing and conflicted behavior.
For the rest of the back-story, we need turn to another passage in Chazal. Antoninus asked his friend Rabbenu Hakodosh about the time that the yetzer hora attaches itself to each new person who comes into this world. Is the yetzer hora created alongside the new being, residing, so to speak, in the new fetus as it develops? Or does it first become part of the new child only when it emerges from the womb as an independent being? The gemara’s conclusion is that the latter is true. Would the yetzer hora enter any earlier, a fetus would rebel at its confinement, and seek to escape it mother’s womb.
This question – coupled with the unusual phenomena associated with her pregnancy- plagued the panic-stricken Rivka. Perhaps, she thought, the yetzer hora of the child I carry indeed had an early beginning in its development. If this yetzer hora is there and already acting out, am I going to find myself victimized by an attempt to prematurely escape, which might easily kill me in the process!
It was this question that – not a search for medical advice – prompted Rivka to seek the counsel of Shem and Ever. Were the bizarre symptoms that she experienced brought on by a very active yetzer hora – in which case, it might easily kill her in an attempt to exit and run? If not – if the yetzer hora simply does not arrive that early – then how to explain the contradictory tendencies of whatever was inside her, stirring to leave in the vicinity of a beis medrash and also a place of idolatry?
HKBH provided the authoritative answer. Rivka – you have nothing to fear from a yetzer hora- driven abortion attempt. The yetzer hora is not a player in your pregnancy. You are most definitely not in mortal danger. The strangeness of your symptoms owes to the most unusual composition of the contents of your womb. While the behavior of the twins seems like they are acting according to their inclinations, this is not because of the active yetzer hora of one of them. The behavior is but a sign, a harbinger of what will play out in their lives years later.
The proof is that at the moment, they are evenly matched. The stirrings are just as likely in front of a shul as near avodah zarah. Know that this will not be the case when they emerge – at which time the yetzer hora of one of them will indeed become active. In “real life,” there will constantly be a struggle between them – but one will always prevail over the other. There will not be balance, but a shifting of power from one to the other. In time, the older will serve the younger. He will presage this battle between them by holding on to the heel of his brother as they emerge into the world.
At the moment of their birth, they will be twins / tumim. The word is related to tam, complete, whole-hearted. They will come in to the world unafflicted by a yetzer hora that developed in utero. In that regard, they will be like everyone else. Your present fears, Rivka, can therefore be allayed.