The Hebrew is vayit’rotz’tzu, which literally means that the babies were crushing one another with the result that Rebekah was suffering terrible discomfort and pain and questioned why she was alive if this was happening to her.
According to the Midrash, they were fighting over which one would get a better portion in life than the other.
Rashi acknowledges this theory but also says that the word might come from larutz, to run. He quotes the well-known belief that whenever Rebekah passed by the bet midrash (study hall) set up by Shem and Ever, Jacob tried to get out of the womb and go and learn Torah (Gen. R. 63:6); but when she passed an idolatrous shrine, Esau tried to get out.
What the rabbinic sages were trying to read into the story was that everything has a prehistory, and the conflict between the two boys began in the womb.
But this antagonism was not just personal but suggested a clash of world-views: Jacob was a sensitive man of intellect and spirituality, whilst Esau was a materialist and sensualist with very little interest in God or the Torah (see Gen. 28:27).
The story of the two brothers illustrates the principle that has often been remarked upon and analysed, that two children brought up in one and the same home can diverge radically in their paths and attitudes, so much so that you can hardly recognise that they come from the same family.