The outrage against me is due to you! (Genesis 16:5).

    Rashi explains the nature of Sarah's complaint against Avraham: "When you prayed to God for a child . . . you prayed only for yourself (and you were granted Yishmael, through Hagar). You should have prayed for both of us, and my desire would have been fulfilled by Him as well!"

    Why indeed did Avraham see fit to omit Sarah from his prayers to be blessed with a child?

    The Rambam (Hil. Berachos 10:22) writes: "When a person is about to measure the volume of his harvest he may pray, `May it be God's will to bestow a blessing upon the work of my hands!' But once the harvest has already been measured this would be a prayer uttered in vain. For anyone who prays for something that has already been determined (such as the amount of his crop, or the sex of an unborn baby) is uttering a prayer in vain."

    The principle formulated by the Rambam may be summed up as follows: Any prayer in which one asks God for departure from the regular course of nature is a prayer in vain. Of course anything is possible for God, and He could change the size of a crop or the sex of a baby after it has already been established as fact. But to do so would require a miraculous intervention in the natural processes of the world, and it is improper to pray for such an occurrence.

    The Talmud tells us, based on Bereishis 11:30, that not only was Sarah barren, but she did not have a womb in her body at all - which placed her conceiving and bearing of a child incontrovertibly within the realm of the miraculous. Avraham, on the other hand, although he was old and beyond the normal age of fathering children, was not absolutely barred by the laws of nature from having a child. For this reason it was still appropriate for him to pray that he should be blessed with a child , but to pray for Sarah, given her physical condition, would have constituted a "prayer in vain."

    -- Brisker Rav