Why is the quickening a poor marker for the start of personhood?
The quickening is the point when a pregnant woman can feel fetal movement in her womb. Some people try to define personhood by movement—“If it moves, it must have personhood.” This is problematic for many reasons. For a striking example, watch a headless chicken fly in the video on this page (WARNING: this video is graphic, and thus age-restricted. To watch the video, click "Watch on YouTube" in the dark box). When we witness a chicken moving without its head, we are forced to reconcile the difference between the continuation of life and the beginning and end of an organism. The moment the head is cut off, the chicken can no longer control its body; however, the headless body of the chicken continues to move and exhibit reflexes until the cells that form its nervous system die. The cell is the basic unit of life, and cells can move. Just because a cell is moving doesn’t mean there is a person in that cell controlling the movement. Likewise, just because a body is moving doesn’t mean there is someone in that body controlling the movement. It's disturbing to think about, but no less true.