2,337. Forbidden Fats From an Animal's Fetus

Maachalos Assuros 7:3

If someone slaughters an animal and finds a fetus in its womb, all of the fetus’ fat is permitted. This is so even if the fetus comes out alive because it is considered to be a like limb of its mother. If the fetus was carried to full term and found alive, then its fat is prohibited and one is liable to kareis for eating it. This is so even if the young animal never stepped on the ground and doesn’t need to be ritually slaughter. In such a case we must remove all the forbidden parts from it the same as we do with other animals.

Maachalos Assuros 7:4

Let’s say that someone sticks his hand into an animal’s womb and removes some of the fat from a fetus that has gone full term. In such a case, one is liable for eating it the same way as he would be if he cut the fat from the mother animal. The is because coming to full term is what triggers the prohibition against eating forbidden fats.