Abraham's wife could not give him a son so he impregnated his slave, Hagar, who gave birth to a son, Ishmael (Islam's Mohammed). The to be continued will probably include that Abraham's wife did conceive a male heir later and that Hagar and her son were sent away and that Ishmael becomes Mohammed, the prophet of Allah.
The point of the post was that Hagar may be the first known "other woman" to man. But a good lesson is that all of these religions...Judaism, Christianity, and Islam...share the same people and the same God. Many Christians don't have any clue about this.
I'm a short worded, to-the-point person, myself, which makes the papers I have to write very challenging. Getting that PhD (a hundred years from now) is going to be a major PITA. LOL