I live in Canada, where abortions after 20 weeks are legal (though not all doctors perform abortions after 20 weeks).
While it's legal, only a very small percentage of abortions are performed after 20 weeks (less than 1%). 90% of abortions are performed before 13 weeks.
The typical reasons why abortions are performed after 20 weeks are:
1. Finding out that the fetus has a severe or fatal health problem.
2. The mother's health or life is in danger.
3. Extremely long wait times (i.e. the woman tried to get an abortion earlier, but wait times pushed it past 20 weeks). --- It should be noted that while abortions are legal, they are not very accessible in many places (for example, in 2008, there were only 67 abortion providers in Texas---92% of counties in Texas had NO abortion provider; and there are some states that have only one abortion provider in the entire state).
4. Not knowing or in denial about being pregnant (more often the case with young teenagers; also to note is that some women have intermittent bleeding during their pregnancy which can be mistaken for their period, or some women have irregular periods, so going months without one is common for them).
From my understanding, the Texas bill did not make exceptions for fetal or maternal health problems. Further, the exceptions for rape and incest acted to punish rape victims, by only allowing abortion if they reported the assault to law enforcement (there are a number of reasons why a victim might not feel comfortable going to the police and they should not be punished for that).
Also to note, the idea that fetuses can feel pain at 20 weeks is highly disputed by mainstream medical associations.