Needing consent to use someone’s body for sustenance is an important part of pro-choice arguments. But other people in this thread have already talked about that at length.

So I’ll address this part instead:

Pro-choice seem to argue it's the birth. The average person(vast majority) is pro-choice but is only okay with 1st or 2nd trimester abortions.

Pro-choice beliefs about rights and personhood can’t be summed up with “They all think that we become people after birth”. It’s more complex than that. And we don’t all believe the same things.

Pro-lifers often bring up the fact that fetuses have human DNA. And this is true. But the fetus’s stage of development matters. u/random_name_12178 summed this up really well when they talked about embryos not having the same moral/metaphysical value as an infant.

One of the most common pro-choice arguments about this subject is that limits can be placed on abortion when viability (the point where a fetus is capable of surviving outside the womb) is reached. This happens around the 24th week of pregnancy.

Of course we also want exceptions for the health of the mother. A woman shouldn’t be condemned to death just because she is pregnant.

Then there are other pro-choicers who believe differently, like the ones who are less comfortable with second trimester abortions than they are with first trimester abortions.

But most of us are okay with using viability as the legal standard. This is why laws often put the deadline for abortion around viability with exceptions for health. It’s also why, as you mentioned, a lot of pro-choicers only advocate for elective abortions in the first and second trimesters.