When Rivka feels something amiss inside her, she goes to ask. In Bereishit 25:22-23 she is told that there are 2 nations in her belly and the elder will serve the younger.
Then, in verse 24, when the days of her pregnancy are complete
וְהִנֵּה תוֹמִם בְּבִטְנָהּ
and behold, there were twins in her womb
I have been trying to figure out why the text includes "V'hinei", and behold. She already knew that there were twins so this would not be a revelation to her, and no one else could know until after verse 25 which describes the births.
When Tamar has twins, the same phrase is used, but there, the presence of twins is presented as a surprise as Tamar had no foreknowledge of the multiple. The text also describes that during the birth (38:28) a hand came out and then went back in after the midwife tied a thread around it to establish birth order -- was she expecting a second baby (if so, how did she know)? But no such process is discussed by Esav/Yaakov.
The Rashbam discusses the word and seems to say that V'hinei indicates a change from what was expected/known, but that would fly in the face of Rivka's knowledge of twins. The sforno seems to say that those standing around already knew that there were twins here (from sefaria, "והנה תומים בבטנה, before they were born, the onlookers already realised that there were twins in her womb.")
So what does v'hinei accomplish in this verse?
