John Calvin Commentary Genesis 30:3

"And she said, Behold, my maid Bilhah, go in unto her; that she may bear upon my knees, and I also may obtain children by her." — Genesis 30:3 (ASV)

Behold my maid Bilhah. Here the vanity of the female disposition appears. For Rachel is not induced to flee to the Lord, but strives to gain a triumph by illicit arts. Therefore she hurries Jacob into a third marriage. From this we infer, that there is no end of sinning, once the Divine institution is treated with neglect.

And this is what I have said, that Jacob was not immediately brought back to a right state of mind by Divine chastisements. He acts, indeed, in this instance, at the instigation of his wife: but is his wife in the place of God, from whom alone the law of marriage proceeds?

But to please his wife, or to yield to her insistence, he does not hesitate to depart from the command of God. To bear upon the knees, is nothing more than to entrust the child when born to another to be brought up. Bilhah was a maidservant; and therefore did not bear for herself but for her mistress, who, claiming the child as her own, thus obtained the honor of a mother.

Therefore it is added, in the way of explanation, I shall have children, or I shall be built up by her. For the word which Moses here uses, is derived from בן (ben), a son: because children are as the support and stay of a house. But Rachel acted sinfully, because she attempted, by an unlawful method, and in opposition to the will of God, to become a mother.