משנה אויף דברים 14:6

If an animal was having difficulty giving birth and the fetus put forth a limb and then put it back in, it may be eaten [when its mother is slaughtered]. If it put forth its head, even though it put it back in, it is considered as born. Whatever is cut off from the fetus within the womb [and left inside] may be eaten, but whatever is cut off from the spleen or kidneys [of the animal and left inside] may not be eaten. This is the rule: that which is from the body of the animal is forbidden, but that which is not from the body of the animal is permitted.