John Calvin Commentary Genesis 21:7

"And she said, Who would have said unto Abraham, that Sarah should give children suck? For I have borne him a son in his old age." — Genesis 21:7 (ASV)

Who would have said unto Abraham, that Sarah should have given children suck? I understand the future tense is used here for the subjunctive mood. And the meaning is that such a thing would never have entered anyone's mind. From this, she concludes that God alone was the Author of it; and she now condemns herself for ingratitude because she had been so slow to believe the angel who had told her of it.

Now, since she speaks of children in the plural, the Jews, according to their custom, invent the fable that because a rumor was spread that the child was spurious, a great number of infants were brought by the neighbors so that Sarah, by nursing them, might prove herself a mother.

As if, truly, this could not easily be known when they saw Isaac at her breast, and as if this was not a clearer and more distinct proof: that the milk, pressed out by the fingers, flowed before their eyes. But the Jews are doubly foolish and infatuated for not perceiving that this form of expression has exactly the same meaning as if Sarah had called herself a nurse.

Meanwhile, it should be observed that Sarah combines the role of nurse with that of mother. For the Lord does not prepare nourishment in vain for children in their mothers’ bosoms before they are born. But those on whom He confers the honor of being mothers, He, in this way, also appoints as nurses, and those who consider it a hardship to nourish their own offspring break, as far as they are able, the sacred bond of nature.

If disease or anything of that kind is the hindrance, they have a just excuse. But for mothers to voluntarily avoid the trouble of nursing for their own pleasure, thus making themselves only half-mothers, is a shameful corruption.