pursuer

A fetus does not pursue the conclusion of the Gemara, so the example is not accurate. Although it is of course possible to ask about this there as well. The example is a small pursuer (which does not merit a card).
In the case of a regular, intentionally large persecutor, it seems simply that this is an obligation. But even in the case of a small persecutor, it seems that it is an obligation to kill him because according to the law it is forbidden for a person to give himself up to save his friend. Regarding a fetus, it seems that perhaps the mother is permitted to give herself up because he is not a persecutor, only his life is less valuable.
But it’s hard to say anything clear here.