When her days … fulfilled: her refers to Rebekah. Days … fulfilled refers to the nine months of her pregnancy being completed. This clause may be rendered, for example, “When it was time for her to give birth” or “When Rebekah was ready to give birth.”

Behold, there were twins in her womb: behold expresses an element of confirmation of what had been said in verse 23. Good News Translation and some other modern translations do not represent this element. Bible en français courant, however, says “When the moment of her delivery arrived, there was no longer any doubt.” Revised English Bible has “There were indeed twins in….” Some other translations say “When the time came it was true that she gave birth to two boys.”

Twins in her womb: it is more natural in English to say “she gave birth to twins” or, as Good News Translation says, “twin sons,” since both were male.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .