Sarai was getting impatient with the whole childbearing thing. It had been about ten years since the covenant with Abram and the delicious masala that surely followed. Sarai had this idea that Abram could father a child with her maidservant, Hagar, and the whole family making business could get a jump start.
I'm pretty sure she didn't consult with Hagar on this matter.
Abram thought it was a pretty good idea for whatever reason, and made Hagar his wife. So now he had at least two wives. And Hagar, not being barren, became pregnant. So this clinches that Sarai was the one shooting blanks, I suppose.
Well, Hagar was a little disappointed about the pregnancy thing. Maybe she was working on her degree and this got in the way of her studies. The author says she 'began to dispise her mistress.' So I take it even though Hagar was now a wife, she still had to work for Sarai. Doesn't seem to be a good deal.
Sarai complained to Abram about Hagar's attitude, and Abram said, I don't care what you do, she's your maidservant, your problem. So Sarai 'mistreated' Hagar (?) and Hagar said, fuck this I'm leaving.
Hagar was from Egypt, so when she ran away, that's the direction she took. She was sitting beside a well at the roadside when god sent a messenger to her. The messenger asked her where she was going.
Hagar probably shrugged and said, well, my mistress, who gave me to her husband for conception, was being generally crappy and insensitive to me so I left.
The messenger told her to go back and endure it. And made the now-familiar promise to Hagar that god would increase her descendents.
The angel/messenger told her that her kid would be called Ishmael, but that he was going to turn out to be a right bastard. I guess that made Hagar feel better somehow, because she said, well, god does pay attention to people other than Abram.
She returned and bore Abram a son and they named it Ishmael.