Reference for Chullin 143:6

אלא אמר רבא

Why is it said 'was not imposed by Biblical law but by decree of the Scribes'? - So that you should not say that our Mishnah agrees [only] with R'Akiba who holds that a [dead] foetus whilst yet in the womb of its mother is unclean;<span class="x" onmousemove="('comment',' Accordingly the midwife, is by Biblical law rendered unclean by reason of contact with the foetus, for a swallowed unclean matter can convey uncleanness; the mother, however, remains clean because the uncleanness touches her in her secret parts and this does not render her unclean.');">⁴</span>

Jastrow