Now Sarai, Abram's wife, had borne him no children. She had a female Egyptian servant whose name was Hagar. And Sarai said to Abram, “Behold now, the Lord has prevented me from bearing children. Go in to my servant; it may be that I shall obtain children by her.” And Abram listened to the voice of Sarai. So, after Abram had lived ten years in the land of Canaan, Sarai, Abram's wife, took Hagar the Egyptian, her servant, and gave her to Abram her husband as a wife. And he went in to Hagar, and she conceived. And when she saw that she had conceived, she looked with contempt on her mistress. (Genesis 16:1-4)
Trying the Hagar option is Sarai's desperate attempt to regain control of an uncontrolable situation instead of trusting God’s promise. If she was right about God preventing her having children so far, why does she think this plan would get round His? God’s plan is for Sarai's good. He is more committed to her good than she is. If he’s preventing something that is for her benefit
That's all very easy to say but difficult to believe when you are in the middle of the situation. It's easy to desperately cling onto a plan, a technique or a superstition that gives the illusion that you are regaining control in the wait for conception. That’s when we need to constantly look back at the story of Abram and Sarai, not (just) to be reminded of her futile attempt at taking matters into her own hands but to see that God’s promise to Abraham was fulfilled in Jesus. When it doesn’t feel like God is good, we see that He is good by looking at Jesus.
Sarai’s attempt at control led to an out-of-control situation with conflict between Sarai and Hagar, Sarai and Abram and, eventually, Ishmael and Isaac. Sarai was never in control of her infertility, and neither are we. Give up the illusion of control and trust the only One who is in control.