Genesis 30:3

And she said
in order to pacify Jacob, and explain her meaning to him; which was, not that she thought it was in his power to make her the mother of children, but that he would think of some way or another of obtaining children for her, that might go for hers; so the Arabic version, "obtain a son for me": but, since no method occurred to him, she proposes one: behold my maid Bilhah, go in unto her,
take her and use her as thy wife: and she shall bear upon my knees;
either sit on her knees in the time of labour, and so bring forth as if it was she herself; or rather bear a child, which Rachel would take and nurse, and dandle upon her knees as her own, see ( Isaiah 66:12 ) ; that I may also have children by her;
children as well as her sister, though by her maid, and as Sarah proposed to have by Hagar, whose example, in all probability, she had before her, and uses her very words; (See Gill on Genesis 16:2).