A new mishnah - a bat Yisrael whose husband is a kohen, and he dies while she is pregnant - a case that raises all kinds of questions about the status that the pregnancy itself changes or doesn't. For example, who can eat terumah may be dependent on the phenomenon of her pregnancy. Which leads into a discussion of the pregnancy, and its impact. Plus, a second mishnah - on the forbidden relationship with the kohen, but from the male side, and the impact shifts to a question of yibum (and still terumah, of course).