What is a fetus at that age? People kill animals way more intelligent than a 20-week old fetus all the time but no politicians cry out for their right to life (think cows, pigs). Most people can stomach the idea of killing these animals because, well, they're tasty and people have to eat to survive. But, as vegetarians and vegans can attest, eating animals for their meat is a choice, not a necessity.
Killing an animal for no reason is immoral, but they are killed all the time because they are tasty.
Killing a fetus for no reason is immoral, but is it immoral to do so if it means sparing a fully-alive woman from bearing to term a thing that is equivalent in many ways to a dumb animal? Why are people so anxious to have another child born into this world? Yes, human life is wonderful, but we don't ask our women to produce the maximum number of babies they can in their lifetime just to have more wonderful people; we let mothers decide how many, if any, they want.
Laws prohibiting abortion are inconsistent with our other cultural values.