When Leah saw that she had stopped having children, she gave her servant Zilpah to Jacob as a wife.
9. When Leah saw that she had left bearing. Moses returns to Leah, who, not content with four sons, devised a method whereby she might always retain her superior rank: and therefore she also, in turn, substitutes her maid in her place.
We have here the bad consequences of that strange marriage which Jacob made with the two sisters. Here is, I. An unhappy disagreement between him and Rachel (Gen 30:1, Gen 30:2), occasioned, not so much by her own barrenness as by her sister's fruitfulness.
Commenting on Genesis 30:1-13
And Zilpah, Leah's maid, bore Jacob a son. For it seems he consented to take her to wife at the motion of Leah, as he had took Bilhah at the instance of Rachel; and having gratified the one, he could not well deny the other; and went in to her, and she conceived, though neither of these things are mentioned, but are all necessarily supposed.