So Abraham prayed to God; and God healed Abimelech, his wife, and his female servants. Then they bore children; for the Lord had closed up all the wombs of the house of Abimelech because of Sarah, Abraham’s wife. (Gen 20,17-18)

Abraham did not just receive protection, compensation, and multiplication of his flocks in Gerar, all in spite of some unbelief on his part. This kind of blessing accompanied him on his journey and he experienced it many times. But now he had a completely new spiritual experience: God had answered his prayer for healing in a very concrete way.

Virtually all ancient cults related to fertility in some way, because it was a guarantee of the continuation of the lineage, of simple survival. Without its own offspring, the tribe had no other fathers, shepherds or warriors. In times when there was no medical security and local tribal wars often broke out, disputes were settled by the extermination of whole families or tribes, a considerable loss of offspring had to be reckoned with, and so only its abundance gave some guarantee for the future.

God "closed...every womb". God was thus demonstrating His power over the deities of Abimelech's tribe (to whom they certainly tried to appeal for help, as they normally did) and confirming Abraham as his own man before them. "Touch him not, or calamities will befall you," he was announcing to them in a way they understood well. All sorts of ideas about how to deal with Abraham were very likely in the minds of Abimelech and his people, so it was necessary to give him additional protection. But what exactly happened with the women? We don't know, but from the general tenor of the text, it is certain that they discovered their barrenness very soon after the incident with Sarah - for it is clear that Abraham prayed for them shortly after her release, not after many months when none of them would have had a child visibly. As much as the people of that time did not have a deep knowledge of medicine, they understood the area of childbearing, for it was so important to them. Therefore, though we do not know for sure, of course, the simplest explanation is that what had happened to the women in Abimelech's house long before had happened to Sarah, namely, that "it had ceased to be with Sarah after the manner of women" (18:11, NRSVA).

Abraham's prayer is the first prayer for physical healing that we find in Scripture. The man of God, the prophet, prays by direct commission from God for the deliverance of the people from the sickness that is caused by their having fallen into God's displeasure. It is interesting, therefore, that unlike other healings in Scripture (e.g., prayers accompanying the preaching of the gospel), this is more of a prayer that accompanies the restoration of grace over a person's life in the sense of James 5:15, "and if he has committed sins, he will be forgiven." Today this kind of prayer is understood and practiced rather rarely.