Not everyone agrees on an exact time, typically the viability of the fetus outside of the womb is the consideration.

This would mean a baby that would be just premature wouldn’t be aborted. As you move back the viability would end up varying for each pregnancy, which is why after a set point doctors are involved. They then make a medical judgement balancing the viability and safety to the carrier.

So there is no hard date. The insistence on getting one simplifies a complicated issue where nuance is important.

I’ve noticed that a lot of anti-abortion laws target doctors, specifically to make the fuzzy nature of the cuttoff difficult.