Actually it is legal to perform abortions in the last trimester if the woman's life is at risk ,or irrepairable damage to a major bodily function will occur if the pregnancy were allowed to continue or if the fetus died in the womb.
Some states also allow late term abortions if the fetus is so malformed it will be stillborn or will only live only live a few hours . In those cases the fetus is considered non-viable or unable to survive even with medical help.
Less than one percent of all legal abortions in the Unites States takes place after 21 weeks gestation and less than .08 percent of a notions takes place after 24 weeks gestation.
They are extreme cases. Such as the cases where the fetus has died in the womb.
The extraction of dead fetus is still called an abortion so a big portion of late term abortions are because the fetus is dead, will be stillborn or would only live for a minutes or hours.
Dr. Tiller helped women whose lives/health were in danger and whose fetuses were non viable by perfoming late term abortions in these extreme cases.
From pages 8 & 9 of the Abortion in Kansas 2008 web site:
[Abortions past the 22 week gestation mark]
Was the fetus viable ? No 131 (they died in the womb...or they would be stillborn.or die within a few minutes or hours ...They were NOT viable)
To prevent substantial and irreversible impairment of a MAJOR bodily function
192 out of 192.
So out of 323 late term abortions that took place 2008 ...
323 were either not viable or continuing the pregnancy would have caused substantial , irreversible bodily damage.
http://www.kdheks.gov/hci/abortion_sum/08itop1.pdf