Kommentar zu Bereschit 21:2

למועד אשר דבר אתו Render it as Onkelos translates: “At the appointed time which He had spoken: — the time He had said and appointed. When He had said to him (18:4) “At the set time I will return unto thee”, He scratched a mark in the wall and said to him (Genesis 53:6), “When the sunrays touch this mark next year she will bear a son” (Yalkut Shimoni on Torah 59:1).

ותהר ותלד שרה לאברהם בן. Sarah became pregnant and bore a son for Abraham. The Torah needed to mention the pregnancy because we had learned that Sarah did not have a womb. This shows that G'd performed an additional miracle for her (Bereshit Rabbah 53,5). The reason the Torah underlines לאברהם is to make it clear that only this son is to be viewed as an extension of Abraham, not Ishmael, as we read later in 21,12. Even though the Torah described Hagar as bearing a son "for Abraham" (16,15), the fact is that at that time Abraham was still called Abram. Once his name was changed, Abraham was no longer to be referred to by his original name. This means inter alia that Ishmael was not a son of Abraham. Perhaps the statement in Berachot 13 that someone who calls Abraham Abram is guilty of violating a positive commandment is precisely for that reason. Our sages do not want Ishmael characterised as a son of Abraham. Such considerations did not apply to Jacob whose name was changed to Israel as he could comfortably identify with all his sons. I have written some more about this subject on 35,10.

ותהר, seeing that the conception had not been mentioned in the previous verse, the Torah now reports conception and birth as if it had been a consecutive activity. We find a similar construction in Genesis 4,1 ותהר ותלד את קין, “Chavah conceived and gave birth to Kayin.”