Legal definition of Self Defense:
The cases for which a man may defend himself are of two kinds; first, when a felony is attempted, and secondly, when no felony is attempted or apprehended.
1st. A man may defend himself and even commit a homicide for the prevention of any forcible and atrocious crime, which if completed would amount to a felony; and of course under the like circumstances, mayhem, wounding and battery would be excusable at common law. A man may repel force by force in defence of his person, property or habitation, against any one who manifests, intends, attempts, or endeavors, by violence or surprise, to commit a forcible felony, such as murder, rape, robbery, arson, burglary and the like. In these cases he is not required to retreat, but he may resist and even pursue his adversary, until he has secured himself from all danger.
2d. A man may defend himself when no felony has been threatened or attempted: 1. When the assailant attempts to beat another and there is no mutual combat, such as where one meets another and attempts to commit or does commit an assault and battery on him, the person attacked may defend himself, and; 2. An attempt to strike another, when sufficiently near so that that there is danger, the person assailed may strike first, and is not required to wait until he has been struck.
I found none of the above applies to a helpless fetus. So killing a fetus is not within the ambit of self defense.