Chullin 71 - Swallowed Impurity
If a woman's fetus died inside her womb, and the midwife extended her hand into the womb and touched it, the midwife is impure for seven days with corpse-impurity, but the woman is pure until the fetus emerges from her womb.
The operative principle here is that of "swallowed-up," or absorbed impurity, which does not transmit impurity to anything else. It is derived from the following phrase in the Torah, " And one who eats of the dead animal carcass shall immerse himself ..." and become pure after sunset. This instruction includes even the case when he swallowed the meat just prior to sunset, and then immersed himself, so that the impure meat has not yet been digested - and he is nevertheless pure, which proves our principle.
Rabbah said that the same is true for a swallowed-up pure object: it does not become impure. What's the proof? Compare the human to a clay vessel, which protects from the impurity from outside. The power of a human is even stronger, since it protects from the swallowed-up impurity.
What about swallowing an impurity via the rectum? Its protection must be even stronger, since the rectum leads to digestion.
Art: G. Horning Jensen - Turning the clay in the pottery