Revākhaṇḍa of the Skandapurāṇa manuscripts (RKS) 88.28

88.28

yāvad vatsasya pādau dvau mukhaṃ yonyāṃ ca dṛśyate /
tāvad gauḥ pṛthivī jñeyā yāvad garbhaṃ na muñcati //

As long as the two feet and the face of the calf are seen in the birth passage,
for that long the cow is to be regarded as the earth, until she releases the embryo.