“Now these are the records of the generations of Isaac, Abraham’s son: Abraham became the father of Isaac; and Isaac was forty years old when he took Rebekah, the daughter of Bethuel the Aramean of Paddan-aram, the sister of Laban the Aramean, to be his wife. Isaac prayed to the LORD on behalf of his wife, because she was barren; and the LORD answered him and Rebekah his wife conceived.” Gen. 25:19-21
We read here that Isaac was forty years old when he married Rebekah, and later in verse 26 that he was sixty years old when Jacob and Esau were born, thus Rebekah was barren for twenty years. This is not the first encounter of barrenness in scripture, as Sarah was barren as well, for at least 25 years (we know Abraham was 75 when he left Haran (Gen. 12:4) and it was already known Sarah was barren before that (Gen. 11:30).
And so we see an increase in blessing and less of a wait for fulfillment; Through Sarah came one child, through Rebekah came twins after a shorter wait. The fulfillment of the abundance of Abraham’s offspring is now beginning; just as Abraham was only able to physically acquire one small plot of all the land promised to him, so too he only sees one of the children that the people of the Covenant would come through.