While it is true that there is not a substantive biological difference, it is also true that it almost never happens that a pregnant person would choose to abort a baby right before it is supposed to be born. If one were to carry a pregnancy nearly to term, it seems safe to assume that they are confident in their choice to do so. Why would anyone sacrifice nearly nine months of their life to grow a human and then suddenly change their mind about it? The majority of abortions occur far before the third trimester and the only reason a baby would be aborted so far along would be because of life-threatening medical circumstances. Moreover, if a baby about to be born were to be removed from their parent's uterus, doctors would be able to keep it alive. No one would abort a baby that is nearly to term.
When it comes to the definition of what determines when a fetus counts as a life, that is definitely a gray area, but it is ultimately up to the person carrying the pregnancy. If anyone thinks that this is ridiculous because they believe that people would be aborting babies in the third trimester, then they have no faith in the empathy and humanity of these people who are carrying pregnancies. It is necessary to trust the judgment of the people who are themselves pregnant, because it is their bodies, their time, and their lives that are being most affected.