Answer

A fetus is not a pursuer, according to the Talmud’s conclusion, so the example is not precise. Though of course one can ask it there as well. The relevant case is a minor pursuer (who is not pursuing her because Heaven is driving him to it).
With an ordinary pursuer—an adult acting intentionally—it seems obvious that this is an obligation. But even with a minor pursuer, it seems there is an obligation to kill him, because according to Jewish law a person may not surrender himself in order to save his fellow. As for a fetus, it seems that perhaps the mother is permitted to give up her own life, because it is not a pursuer; rather, the value of its life is lower.
But it is difficult to say anything definitive here.