Changing the legal definition of personhood from birth to fertilization would have major effects in many areas. Pregnant women would face more control over the choices they make during their pregnancy. Their actions might be judged for how they affect the fetus which could limit normal actions like what they can eat, what medicines they take, how they exercise, and more. Some types of birth control, like IUDs or emergency pills that stop fertilized eggs from attaching might be banned since by this logic, they could be seen as ending a life. Fertility treatments could also become more complicated. These often create several fertilized eggs, and not all are used. If each one is legally a person, freezing or discarding them could be illegal. Abortion, both planned and unplanned would legally be treated as ending a person's life no matter the reason or timing. In my opinion, I don't think this change makes sense for a few reasons. It can cause many issues and confusion around healthcare, it takes away the mothers autonomy in a much bigger way, and it fails to acknowledge the many fertilized eggs that don't make it into the pregnancy stage.