Why Sarai Chose Hagar
What's behind the whole business of Sarai (Sarah) telling her husband Abram (Abraham) to have a child with her maidservant?
Women in ancient times only gained status as a result of marriage or childbearing. Not having a child was a huge negative mark on a woman's character. Barrenness was always attributed to the woman, not to the man. So, Sarai, now around 75, was ready to have a child. But that wasn't happening.
It has been ten years since God first told Abram, "Go from your country and your kindred and your father's house to the land that I will show you. I will make of you a great nation, and I will bless you, and make your name great" (Gen 12:1, 2 NRSV).
During these ten years, God has clarified the details of the blessing not once, but twice: God explains, "I will make your offspring like the dust of the earth" (13:16). Still waiting for even one of those many offspring, Abram finally tells God that he's considering adopting his servant Eliezer of Damascus to be his heir. God responds, telling Abram that the offspring he'll have will be his own "issue" (15:4).
Interestingly, there's no mention of Sarai giving brith to these offspring. It seems only human that Sarai would be getting impatient, wondering just how the offspring were going to be produced.
In those days in the Near East, there were provisions for using a surrogate to provide the barren wife with a child for her husband. The surrogate could easily be the wife's slave or servant, as it was with Sarai. The wife would then adopt the child as her own. Her husband would have an heir, and they would be taken care of by their child. (Sarna 119, 113)
Perhaps Sarai was thinking along such lines and believed that giving her Egyptian maidservant, Hagar, to Abram was a way, or the only way, to give Abram the "offspring" God had promised him. Perhaps she was truly frustrated by being barren. Perhaps she was just tired of waiting for God and felt God needed some help. We probably won't ever know.
But we do know that Sarai takes matters into her own hands. She doesn't wait for God. She goes with a custom of surrogate mothers that we find recorded only twice in the Bible -- with Abram and Sarai and with Jacob and Rachel and Leah (IVP 48).
So how does it come about that Abram has a child with Sarai's slave? Sarai tells him: "'You see that the Lord has prevented me from bearing children; go in to my slave-girl; it may be that I shall obtain children by her. And Abram listened to the voice of Sarai…. He went in to Hagar, and she conceived" (Gen 16:2, 4). Sarai has an idea, presents it to Abram, and Abram goes along with it. Neither of them knows that God is planning for the two of them to have a child in about another 15 years.
It is very interesting to note that Abram obviously values his wife and her opinions. What happens as a result of this decision is a different Q&A.
Walton, John H., Victor H. Matthews, Mark W. Chavalas. The IVP Bible Background Commentary: Old Testament. Downers Grove, IL: InterVarsity P, 1997.
Sarna, Nahum. JPS Torah Commentary: Genesis. Philadelphia: Jewish Publication Society, 1989.