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.
John W. Ritenbaugh
Eating: How Good It Is! (Part Two)
Women who have carried a child can relate to morning sickness and fetal movements, but what Rebekah experienced with these two fetuses engaging in wrestling matches in her womb is probably beyond our comprehension! Having an upset stomach or being somewhat queasy just does not compare with the abnormal amount of jostling and grappling the unborn brothers were doing. It grabbed her attention!
Richard T. Ritenbaugh
All About Edom (Part One)
Other Forerunner Commentary entries containing Genesis 25:21: