Isaac was her son. His mother sent him away to Syria to find a wife and he did, Rebekah. After working years and years he won the favor of her father to marry her. But she too was barren. No children. Again the promise seemed like a night of bad pizza. But God heard their prayer and opened up her womb. Twins even.
Next enter Jacob. Rachel was barren. Would this family curse ever end? All these beautiful women they married were barren. Once again it seemed as if the promise would not be fulfilled. But her sister bore children, and then so did she.
At every step the promise faced impossibilities for it to be fulfilled. Impossible save for a miracle of God. But nothing is impossible with God, even if his multiple miracles were needed for every generation.