An excerpt from "Bible Tools.org" entitled, "What the Bible says about Rebekah, God's Revelation to" by John W. Ritenbaugh, states: "God's revelation to Rebekah regarding the struggling twins is that two kinds or types of people were in her womb. They were definitely not identical twins. The word "manner," as used in the King James Version, indicates the reason for their rivalry; they were so different despite having the same parents. Their struggling in Rebekah's womb was a precursor of what continued after their birth, which significantly influenced the history of Isaac's descendants.
Each son's approach toward and manner of life irritated the other. Each rubbed the other the wrong way. Rebekah seems naturally drawn to Jacob and Isaac to Esau, exacerbating an already volatile situation. Thus, each boy became a victim of the parent's favoritism and was encouraged to take advantage of it." This article may be viewed at: What the Bible says about Rebekah, God's Revelation to/bibletools.org
Reference: Romans 9:11