Romans 9:8-10

New Catholic Bible

⁸ In other words, it is not through physical descent that people are regarded as children of God. Rather, the children of the promise are those who are counted as descendants. ⁹ For this is how the promise was worded: “About this time next year I shall return, and Sarah will have a son.”

¹⁰ And not only that, but Rebekah became pregnant by one man, her husband Isaac.