taking matters into their own hands...

Genesis 16


¹  Now Sarai, Abram’s wife, had borne him no children. But she had an Egyptian slave named Hagar; 2 so she said to Abram, “The LORD has kept me from having children. Go, sleep with my slave; perhaps I can build a family through her.”
Abram agreed to what Sarai said. 3 So after Abram had been living in Canaan ten years, Sarai his wife took her Egyptian slave Hagar and gave her to her husband to be his wife. 4 He slept with Hagar, and she conceived.

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APPARENTLY it was not uncommon in this culture for a wife to find a 'surrogate' to carry a child and have authority over them.  So perhaps Sarai's action was not as outlandish as it may seem to us.  She suggests that Abram sleeps with slave girl Hagar, so that through Hagar they may have children.

In spite of the statement in the previous chapter where 'Abram' believed Yahweh's promise of land and descendants, here Abram and Sarai attempt some D.I.Y.  Upon Sarai's request, Abram does what was culturally unremarkable - and sleeps with the appointed surrogate.

But the plan had never been meant to be unremarkable.  God was planning a remarkable thing.  But in the face of their circumstances, they try and do things their way, not God's way.  Sarai places the blame for her barrenness at Yahweh's door (v2) but seemingly does not expect him to change this.  God has made her barren, hence barren she will always be, and another woman is needed in order for a child to be born to them.

And so the plan is put into action, and Hagar conceives a child.