C. F. Keil and Franz Delitzsch on Genesis 25:21

Gen 25:21

Isaac’s marriage, like Abraham’s, was for a long time unfruitful; not to extreme old age, however, but only for 20 years. The seed of the promise was to be prayed for from the Lord, that it might not be regarded merely as a fruit of nature, but be received and recognised as a gift of grace. At the same time Isaac was to be exercised in the patience of faith in the promise of God. After this lengthened test, Jehovah heard his prayer in relation to his wife. לנוכח, Gen 25:21 and Gen 30:38, lit., opposite to, so that the object is before the eyes, has been well explained by Luther thus: quod toto pectore et intentus in calamitatem uxoris oraverit. Sicut quando oro pro aliquo, propono illum mihi in conspectum cordis mei, et nihil aliud video aut cogito; in eum solum animo intueor.