Commentary for Genesis 16:3

ותקח שרי AND (SARAI TOOK [HAGAR] — She took (won her over) by kindly speech saying, “Happy are you in that you will be privileged to consort with so holy a person as this” (Genesis Rabbah 45:3).

AT THE END OF TEN YEARS. This is the established period for a woman who has lived with her husband for ten years without having given birth to children, after which he is bound to take another.

ותקח ... מקץ עשר שנים, ten years had elapsed since Avram and Sarai had moved to the land of Canaan. She had waited all these years thinking that G'd would give Avram a son from her in that land. After all, the instruction to Avram to go forth from his birthplace, etc. had been followed by the promise that there G'd would make him into a great nation. (Genesis 12,2). Our sages in Yevamot 64 state that if someone was married to a woman for 10 years during which she was unable to bear a child for him he is not allowed to simply forego the chance of having children, but should pay her out her marriage settlement (Ketuvah) and divorce her as it is possible that G'd only decreed that he should not have children from that particular woman. The Talmud, while admitting that there is no proof for the above, claims that Avram's example can serve as a guide for such conduct, seeing that he waited ten years after settling in the land of Israel, having discounted the earlier years during which Sarai had not conceived.