Why not Quadmesters?

When I discuss abortion with people one of the things that I find most interesting in many pro-choicers is the reliance on trimester framework created by Harry Blackmun in Roe v. Wade. Many people who are pro-choice say that abortion should be legal in the first two trimesters but illegal in the third or legal in the first but then illegal in the second and third.

One way of dealing with this that I've been thinking up is asking them something like, "Why not legal in the first three quadmesters but illegal in the fourth quadmester?" or "Why not legal in the first semester but illegal in the last semester?" or "What would you say to someone who thinks abortion should be legal in the first two pentamesters but illegal in the last three pentamesters?" and so on.

These questions might force individuals who are pro-choice to think about why pregnancy and the legality of abortion should be divided by 3 trimesters and not 2 semesters or 4 quadmesters or 5 pentamesters, and so on. How is the trimester framework any more objective than my quadmester framework?

It's quite obvious to most prolifers that the grounding for the trimester framework is completely arbitrary. There is nothing that magically changes in the unborn child from the last day of the first or second trimester to the first day of the second or third trimester that makes the unborn child more worthy of legal protection. The child is merely one day older. Yet the trimester framework has been used for so long that it is almost seen as set in stone that pregnancy and the legality of abortion should be divided by the number three.

Asking "Why not Quadmesters?" might get some pro-choice people with trimester beliefs to start wondering what their stance on abortion is actually based on. They might eventually discover that instead of objective facts their position is based arbitrary time frames, what they believe is popular opinion, and nothing but "penumbras, formed by emanations."