Reference for Chullin 143:7

פומבדיתאי ידעי טעמא דהא מילתא ומנו רב יוסף

for indeed it is even in accordance with R'Ishmael who holds that the [dead] foetus whilst yet in the womb of its mother is clean, yet here the uncleanness [to the midwife] was imposed by Rabbinic decree.

Jastrow