Sarai, Rebekah, and Rachel could not have children at first (25:21; 29:31). Sarai’s case especially shows the tension between what people could see and what God had promised. God had promised to make Abram into a great nation (12:2).
Israel began through women who could not bear children. This shows that Israel existed because of God’s power and choice. God gave children to women who were barren (unable to have children), as these passages also show (see also 1 Samuel 1:2; 2:5; Psalm 113:9; Isaiah 54:1).