I've posted this
article from the New England Journal of Medicine here a while ago.
It's mostly concerned about neonates feeling pain a birth, but it also gives a very good overview of the development of the human brain in respect to feeling pain.
Basically it says that a fetus develops the physiological ability to feel pain starting with around week 20. That still doesn't mean that there is already a "self" there at that time to actually experience the pain, but at least the necessary nerve connections and brain structures develop around that time.
So judging by this, I'd argue that abortion does indeed become more and more problematic after this stage. So I'd be fine with banning abortions after week 20, as are a lot of other pro-choice people I know.
Films of fetuses supposedly screaming of fighting during an abortion depend purely on a special interpretation of the fetus' movements. They are intentionally missleading and certainly no help in making a sensible decision about abortion.