Less than 100 abortions happen in third trimester and they are extreme cases to save the woman life or irreparable damage of a major bodiliy function. ( stroke, heart attack, paralysis from neck down, kidney damage etc.)
It is not that pro choice is unwilling to comprise on a timeline of 16 weeks as long as abortions for fetal or maternal health are made.
Abprtion and the right to privacy is a Supreme Court decision.
The only thing that would reduce the timeline of abortions to 16 weeks would be if viability was reduced to 16 weeks. ( experts agree that it highly unlikely ,so highly unlike it is near impossible that any preemie younger than 21 weeks gestation would ever survive. Their lungs and digestive are so undeveloped no technology could help them survive.)
The fact is currently over 91 percent of US abortions occur during the first trimester.
Over 60 percent occur under 8 weeks gestation in the US.
Almost all of the abortions past the first trimester are because of fetal or material health.
In fact even though elective abortion is legal in the US until viability our rate of abortions based on percentage of pregnancies was less in 2013 than France, Denmark, or Sweden , and tied with the United Kingdom.
Percentage of pregnancies aborted by country (listed by percentage)