Genesis 30:3

Behold my maid Bilhah. Rachel’s proposal, which Jacob accepted and carried out, was as sinful as that of Sarah (ch. 16:2), but without Sarah’s excuse, since there was now no question as to an heir for Jacob. If such a reason had, indeed, existed, it would not have justified the act, which even in the case of Abraham had been so clearly condemned.

She shall bear upon my knees. This statement has been considered by many commentators as a Hebrew idiom expressing adoption (see ch. 50:23). It is possible that the expression originated in an ancient Oriental custom, whereby, at the birth of a child that was to be adopted, the one adopting the child would receive it as his own. Rachel probably had one of these customs in mind and planned to receive the baby from birth as her own.