Genesis 21:2

And she conceived, and Sarah bare to Abraham a son, who was like to himself in his age, at the time of which the Lord had spoken to him.

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.”

לאברהם, as G’d had said “אשר תלד לך שרה,” “whom Sarah will bear for you.” This is a customary form of syntax, as for instance in 16,15 ותלד הגר לאברהם בן, or Exodus 21,4 וילדה לו בנים ובנות, and many similar examples. A woman is considered similar to the earth in this respect, i.e. just as the earth yields fruit for its owners, so a woman produces children for her husband after he had planted his seed in her. The child was named after its father.

לזקוניו, the Torah writes this so that we better understand why Sarah had laughed when she had heard the prophecy in 18,12 and had said that her husband was too old to sire children. Another reason for the expression לזקוניו is to inform us that whereas Sarah had regained her youth at that time and had ovulated again after having ceased to do so for many years, Araham did not experience any rejuvenation, but sired a child in spite of not shedding signs of old age. There are some words in the Hebrew language which always appear in the plural mode, זקונים is one of them, so are נעורים and בתולים.

אשר דבר אותו, the word אותו here is the same as if the Torah had written עמו, “with him.”

ותהר ותלד, “she conceived, became pregnant and gave birth.” The reason why both stages are mentioned is that just as conceiving-becoming pregnant was painless, so her giving birth was also painless.