In a Sicha from 5722, the Rebbe posited the following question: why is it that only a non-Jew who commits abortion is chayiv misa while a Jew who does the same act is not?
The following is a short synopsis of the Rebbe’s answer:
The existence of a Yid is not a means to an end, to accomplish something within the world; rather, a Yid himself, his very being, is the עיקר and purpose of his existence. Everything else in this world, non-Jews included, their very existence is a טפל for the continued well-being of the Yidden.
A fetus is a life with its potential and purpose yet to have been played out; its current status is טפל to its future self where it will live out its purpose. Based on this we can understand the difference in ruling between a Yid and a non-Jew in the case of abortion. A non-Jew—whose very existence is בגדר טפל— who has an abortion is chayiv misa because the fetus holds just as much of a qualified status as he does, a טפל. On the contrast Yid—being an עיקר— who would go through with an abortion would not be chayiv misa because we would not be מבטל an עיקר for a טפל.
See full Sicha here: לקו"ש ח"ב עמוד 602
The Halacha varies in context of levels of danger to the mother and stage of the pregnancy.
Therefore, it is imperative to consult a Rov in all cases, whether is seems to be a case where one should be maikel or machmir.