In the Genesis lectures, Luther discusses the trial Isaac and Rebekah endured in their infertility.

This is another trial. After the flame of lust has ceased and Isaac has become a husband and has had Rebecca as his wife for 20 years (for so long does God delay the promise in which He had promised his father Abraham: “Through Isaac shall your descendants be named”), another affliction now follows, and indeed one that is far more burdensome than the previous trial. The victor over lust overcame the devil by his chastity up to the time of his marriage. In the marriage state he longs for offspring, in accordance with the promise; and he certainly has no slight hope, since he knows that his wife was prepared for him and brought to him in accordance with God’s plan. But Rebecca does not bear a child; nor does she have a promise that she will be a mother, just as Sarah, too, did not have a promise at first. This undoubtedly troubled his heart, and to this trial were added fear of and worry about perpetual barrenness, which they considered to be a curse. For the fathers laid very great stress on this statement (Gen. 1:28): “Be fruitful and multiply.” They felt that a special blessing of God rested on this statement; and because they did not multiply, they supposed that they were cursed and under God’s wrath.

-Luther, AE 4