Genesis 30:9

When Leah saw. Leah, accustomed to bearing a son each year, became impatient when it seemed she was to bear no more. That Rachel had obtained sons by her maid did not disturb Leah so long as there was prospect of her having sons of her own, but now she became the victim of envy, as her sister had been before. The means employed by Rachel to retain Jacob’s favor made Leah jealous, and jealousy drove her to the employment of the same means Rachel had used. However, Leah seems to have been conscious that she was pursuing a device of her own heart, since she made no reference to God in her statements at the birth of Zilpah’s two sons.

As for Jacob, it is surprising how easily he consented to the devious suggestions of his wives in order to increase their offspring. If he had felt some excuse for taking Bilhah in order to satisfy his beloved Rachel, who had no children of her own, by what excuse could he have quieted his conscience in regard to the proposal of Leah, who already had four sons? Having entered the path of wrongdoing, he seems neither to have seen the wrong of his conduct nor to have thought of its possible consequences. On the other hand, it must be admitted that in so doing he followed a common custom of his day. From the law code of Hammurabi and other cuneiform documents we know that such a practice was legally and socially acceptable, particularly when sterility prevented childbearing. The existence of this custom is probably the main reason that neither Abraham nor Jacob saw any great mistake in taking their servants as concubines.