Midrash su Geremia 18:3

R. Huna, however, maintained in the name of R. Yosé: Though we have been taught that the husband of a pregnant woman who prays “May it be Thy will that my wife bear a son” is uttering a vain prayer, this is not so. In fact, he may pray for the birth of a son even as she commences labor, for it is not difficult for the Holy One, blessed be He, to convert females into males and males into females. He quoted a verse from Jeremiah to support this statement: Then I went down to the potter’s house, and behold, he was at his work on the wheels. And whensoever the vessel he made of the clay was marred in the hand of the potter, he made it again another vessel, as it seemed good to the potter to make it (Jer. 18:3–4). And did not the Holy One, blessed be He, say to Jeremiah: O house of Israel, cannot I do with you as this potter? (ibid., v. 6).