“go in to my slave girl.” “Go in to” is a common and graphic idiom for sexual intercourse (Gen. 30:3; 38:8). It was a common practice in the ancient Near East that a woman who could not get pregnant would have children through a surrogate mother that was a slave. From a man’s point of view, if he wanted children he could just take a second wife, but then the first wife would not have control over those children; they would belong to the second wife. However, if the surrogate mother was the wife’s slave girl, the wife would have control over the children. Although this practice may seem strange to us today, in a time when there was no police force to protect people, and no government that would support people in their old age, having a large family, and especially sons, was the best way to assure having protection and support (cp. Ps. 127:3-5).
Hagar apparently got pregnant very quickly. Abraham came into the Land when he was 75 (Gen. 12:4), and had lived in the land 10 years when he had intercourse with Hagar (Gen. 16:3), so he was 85 years old. The following year, when Abraham was 86, Hagar gave birth to Ishmael (Gen. 16:16). Sarah was ten years younger than Abraham (Gen. 17:17), so Sarah was 75 when she gave Hagar to Abraham so she could have a child. Since she had been so many years without a child, at her age asking to have one by her slave girl seemed like a reasonable request. At the time this request did not seem like it was breaking any promise God made to Abraham. God said Abraham would have children, but in the biblical culture of the time, this was a way to have children, and Ishmael was indeed Abraham’s son. It was only later that God said specifically that Sarah would have a son (Gen. 17:16), and when He did tell that to Abraham, it is likely that Abraham thought that meant Ishmael would die or be killed, because he said, “Oh, that Ishmael might live before you!” (Gen. 17:18). As it turned out, both boys grew and founded nations, but the Messiah came through Isaac, Sarah’s child.