I am also pro-choice - I have thought the viability standard was always going to be a moving target thanks to
medical and technological advances that could topple at some point too - even if you defined it as something like NNth percentile.
It does seem to me that setting an arbitrary (but flexible) point of no return was going to be necessary (probably around 18-24 weeks)
The trimesters have absolutely 0 actual meaning here, as this is fundamentally putting an arbitrary line on a continuous process. Yet this is probably the only way you're getting anything, as they're at least a useful Schelling point that can get some compromise.
It does seem to me that setting an arbitrary (but flexible) point of no return was going to be necessary (probably around 18-24 weeks)