I think that the argument for a cutoff date for abortion mostly pertains to the fact that at some point in the pregnancy the fetus is undeniably human. It looks like a little human and reacts to external stimulus in ways that indicate it can feel pain. That happens disturbingly early in the pregnancy which supplies pressure for early cutoff dates. Certainly, only the disingenuous could deny that at the 3rd trimester. https://www.onhealth.com/content/1/fetal_development_stages