The viability of a living thing is the key to understanding the "value" of that living thing. a fetus of a human is not viable outside its month before about 26 weeks even at 30 weeks it is at extreme risk and is only save by medical heroics and often suffers life long deficits as a result. In nature young that have not fully developed almost always die.
I am pro-choice the health mental well being of a women takes priority of that of a non-viable fetus and over that of a viable one as well in medical cases where a choice must be made.
This, being said in cases of murder or man slaughter where the life of a wanted fetus(child) was taken it is murder or man slaughter as it should be.
So, far animals have limited rights under the law.