[frompom]

Ultimately we need to pick some point. Someone doesn't undergo some transformation upon becoming the age of a legal adult on midnight when becoming of age. Nor does age of consent suddenly confer reasoning capabilities that didn't exist the day prior. Legal definitions don't do well with nuance, we end up having to define thresholds that up close seem ridiculous. Whether it's a certain number of weeks, or upon birth, up close it would always look ridiculous like any other threshold.

In your example 8.5 months vs 9.5 months they may have actually have been conceived quite close to each other as well since gestation time is not actually based on date of conception but date of the last menstruation. So this, as well, is a relatively arbitrary measurement.

It's the reality of having to have universal absolute measures and terms in a world of edge cases and grey areas.

Being a thinking/aware person wouldn't happen suddenly, it would be gradual. Gradual rights would be damn near impossible to codify, so like everything else we have to pick some point that feels reasonable and yet will feel silly and arbitrary at the same time.