An argument I’m toying with here.
A creature is entitled to moral consideration only if it possesses sentience. Sentience is somewhat mysterious, but pro-choice individuals generally believe that fetuses don’t possess sentience, which is why they have no claim to moral consideration.
In contrast, many animals do possess sentience (cows, etc.) For this reason, many vegetarians believe animals have a claim to moral consideration, and therefore ought not to be killed merely for the pleasure of humans.
Thus, fetuses are not sentient, and therefore not entitled to moral consideration; cows are sentient, and therefore are entitled to moral consideration.
Premise: If x kills a being worthy of moral consideration without good reason, then x has committed a grave moral wrong.
Premise: If x kills a being that is not worthy of moral consideration, then x has not committed a grave moral wrong.
Premise: In most cases in modern societies, there is no good reason to kill animals for food.
Premise: Therefore, the killing of sentient animals for food is a grave moral wrong.
However, a fetus is not a sentient animal. Therefore, killing a fetus is not a grave moral wrong.
It follows, then, that killing a fetus for food is also not a grave moral wrong. This doesn’t mean that there aren’t other reasons that stand against killing and eating fetuses (aesthetic, practical, etc.) But it does mean there is nothing morally wrong with chowing down on a fetus, at least absent further assumptions.
Therefore, pro-choice vegetarians should at least be willing to eat fetuses, in a way they aren’t willing to eat cows and other sentient animals. Certainly, they could not condemn someone who chose to eat a fetus. The practical reasons that oppose eating fetuses in most circumstances are insufficient to ground the hostile reactive attitudes we usually deploy against those who mistreat beings worthy of moral consideration.
Arguably, infants also lack sentience, although the case is more difficult to make. For similar reasons, then, pro-choice vegetarians should at least be willing to consider eating infants; and, given the choice, they should probably be more willing to eat human infants than they are to eat cows.