When Leah saw that she had left bearing, she took Zilpah her maid, and gave her Jacob to wife.

Wesley's Notes

30:9 Rachel had done that absurd and preposterous thing of putting her maid into her husband's bed, and now Leah (because she missed one year in bearing children) doth the same, to be even with her. See the power of rivalship, and admire the wisdom of the divine appointment, which joins together one man and one woman only. Two sons Zilpah bare to Jacob, whom Leah looked upon herself as intitled to, in token of which she called one Gad, promising herself a little troop of children. The other she called Asher, Happy, thinking herself happy in him, and promising herself that her neighbours would think so too.

Matthew Henry's Concise Commentary

30:1-13 Rachel envied her sister: envy is grieving at the good of another, than which no sin is more hateful to God, or more hurtful to our neighbours and ourselves. She considered not that God made the difference, and that in other things she had the advantage. Let us carefully watch against all the risings and workings of this passion in our minds. Let not our eye be evil towards any of our fellow-servants, because our Master's is good. Jacob loved Rachel, and therefore reproved her for what she said amiss. Faithful reproofs show true affection. God may be to us instead of any creature; but it is sin and folly to place any creature in God's stead, and to place that confidence in any creature, which should be placed in God only. At the persuasion of Rachel, Jacob took Bilhah her handmaid to wife, that, according to the usage of those times, her children might be owned as her mistress's children. Had not Rachel's heart been influenced by evil passions, she would have thought her sister's children nearer to her, and more entitled to her care than Bilhah's. But children whom she had a right to rule, were more desirable to her than children she had more reason to love. As an early instance of her power over these children, she takes pleasure in giving them names that carry in them marks of rivalry with her sister. See what roots of bitterness envy and strife are, and what mischief they make among relations. At the persuasion of Leah, Jacob took Zilpah her handmaid to wife also. See the power of jealousy and rivalship, and admire the wisdom of the Divine appointment, which joins together one man and one woman only; for God hath called us to peace and purity.