In a post-Roe world, legislators—most of whom have no medical expertise and cannot get pregnant themselves—in one state could assert that life begins at fertilization, while a neighboring state could say life begins at birth.
Next, all hormonal contraception and IUDs might be outlawed just because some people believe they interfere with implantation and therefore “endanger” the new “life.” Defining personhood as starting at fertilization also has significant implications for in vitro fertilization, a process that creates fertilized eggs.