Or HaChaim on Genesis 17:5/אור החיים על בראשית י״ז:ה׳

1 ולא יקרא שמך עוד אברם, your name will no longer be Abram, but Abraham. Bereshit Rabbah 44,10 already stated that when Abram cited that he had seen in his horoscope that he would not father any children, G'd agreed with him and told him that he was correct, that as Abram he would not father children but that as Abraham he would. The same would apply to Sarai as compared to Sarah. This Midrash is very difficult since the Torah reports Abram as siring Ishmael, not Abraham!

2Besides, why did the Midrash mention that Sarai did not bear children when Abraham had never even raised the problem of Sarai not having children?

3We must conclude therefore that Abraham already knew at that time or at least surmised that any meaningful seed of his could only be conceived through Sarah because (presumably) whatever he had foreseen in his horoscope applied to himself in conjunction with his wife. G'd confirmed this when He told him that his seed would be known through Isaac. This is why the Midrash included the name Sarah when describing a prophecy primarily directed at Abraham. Abraham realised that the need for Ishmael to be his son was to rid his sperm of any residual contamination of the original serpent. Fathering Ishmael then was not fathering in the true sense of the word.