Is it obligatory for a woman who miscarried her fetus in the fourth month due to an exertion she was unaware would cause the miscarriage to fast for two consecutive months as expiation for unintentional killing, and what should she do if she gets her menstrual period during the fasting?

If what you did does not normally harm the fetus, then there is no blame upon you. However, if it does harm it, such as carrying heavy objects, then causing a miscarriage in the fourth month is not considered murder, because the soul has not yet been breathed into it. Therefore, no blood money (diyah) or expiation (kaffarah) is incurred, but rather sin and the obligation of repentance. As for someone who began fasting two consecutive months and then experienced menstruation, she should break her fast and then complete her fasting after becoming pure.