Ramban on Genesis 25:23/רמב"ן על בראשית כ״ה:כ״ג

1TWO NATIONS ARE IN THY WOMB. The intent of this is that He informed her that she should not fear, for the reason that the struggle in her womb is that she is pregnant with twins, this being the customary way among women who are pregnant with twins.It is possible that He is also saying that since they are destined to be two peoples, hating and warring with each other, at the very beginning of their creation they initiated a quarrel, thus intimating at the situation which will ultimately exist between them. But He assured her that now they will rest, and she will find rest and quiet for herself.