True, I agree that's a possibility. Plotwise it would make good sense for Euron to do something like that.
Although I have some doubts as well, because if he were going to use her and her fetus as a live sacrifice he should try to keep her alive and in decent shape as long as possible (sort of what Melisandre used to do, prep her victims before the flame or leech). Mutilating a pregnant woman and tying her to the prow of a sailing ship makes me think he doesn't care whether she miscarries/dies or not, which would indicate he may not be much into the blood sacrifice option. Of course Aeron Damphair may be his backup in case she dies.