If an animal is pregnant and you slaughter it, what is the status of the fetus? Does the stage of development matter like in regards to eggs?
The mishna in the fourth chapter of Chullin covers this topic and the gemara (Chullin 68a and ff) develops it. As a summary, R Steinsaltz writes that when a pregnant animal is slaughtered its fetus becomes permitted together with all of the rest of the animal’s internal organs.
In any case, it is clear from the Mishnah that in such a case the fetus becomes permitted as a result of its mother’s slaughter; in the event that the fetus’ head came out it is considered to have been born, and as an independent entity would not become permitted by means of its mother’s slaughter. [...]
Rabbi Yochanan teaches that if the slaughtered animal was opened and that the fetus had not developed into an animal, rather it has the appearance of a dove, it is forbidden. The Gemara explains that this law is based on the fact that the Torah requires that an animal have split hooves if it is to be eaten (see Devarim 14:6). At the same time, if the fetus has the form of an animal – even without split hooves – Rabbi Shimon bar Yochai teaches that it is deemed kosher based on the fact that the same passage talks about a single hoof.
See also here.