Комментарий к Берешит 25:24

וימלאו ימיה AND WHEN HER DAYS TO BEAR WERE FULL — But in the case of Tamar it is written, (37:27) “And it came to pass at the time of her travail”, for her days were not fulfilled as she gave birth to them (her children) at the end of seven months.

והנה תומים whenever the Torah speaks about something new it uses the expression והנה in connection with this. Thus we find it in 29,25 where Yaakov is reported as finding that he had slept with Leah instead of with Rachel as he had thought. In 41,7 we also find the expression הנה when Pharaoh awakens from what he thought was a factual experience only to find that it had been a dream.

והנה תומים בבטנה, before they were born, the onlookers already realised that there were twins in her womb. Therefore, when the first one emerged being completely covered with hair, this was a birth which was presumed to be more difficult than the passage of Yaakov whose skin was smooth and did not cause friction with the lining of the womb, they called the first one עשו, attributing his easy passage to his twin brother who had helped push him out of the womb of his mother. [I believe that the author understands the plural ending in עשו, as equivalent to assu, i.e. the result of a combined effort by both him and his brother.]