Come and see the contrast between the power of the Holy One, blessed be He, and that of mortal man. A man might put his things in a sealed purse whose opening is turned upwards, and yet it is doubtful whether they would be preserved or not; whereas the Holy One, blessed be He, fashions the embryo in a woman's internal organ that is not sealed and whose opening is turned downwards, and yet it is preserved.

(Talmud, Niddah 31a)