Legal abortions in the third trimester ( past 24 weeks gestation) are extremely rare and are to save the life of the woman.
From an old Fox News article ( from 2003 )
Fast Facts: U.S. Abortion Statistics | Fox News
The stats since then have changed.
By 2012 the numbers of abortion were less than 1.2 million.
In 2014 the CDC reported less than I million.
Fewer than I.3 percent of legal abortions took place between 21 weeks gestation and 24 weeks gestation.
Some for maternal health , more for fetal health since fetal abnormalies usually do not show up until the 18-20 gestational week ultrasound. That only gives the parents with input from the doctor and tests to decide if they want to terminate the pregnancy since viabily usually occurs between 22 and 24 weeks.
Less than 100 abortions occur after 24 weeks gestation.
They are the extreme cases.
There are only 3 clinics in all the USA ( and 4 clinic doctors ) who perform legal abortions after the age of viability ( between 22 to 24 weeks ) gestation and they are for the extreme cases. Cases where the fetus is not viable ( will be stillborn or is so malformed will only live a few minutes or hours). If a fetus dies or is dying within the womb , it can case a life threatening infection in the woman. Or if irreparable damage to a major bodily function ( such as stroke, heart attack, paralysis from the neck down , kidney damage etc. ) would occur if the pregnancy continued.
Doctors all of the US who patients had these extreme conditions would send there patients to one of these clinics.
In 2008 Kansas had one of the clinics for these extreme cases.
Kansas kept a record of all abortions at or after 22 weeks gestation.
There were 323 abortions for these extreme cases.
131 were because the fetus was not viable. ( was stillborns or so malformed it would live only a few minutes or hours).
The other 192 were because irreparable damage to a major bodily function would occur if the pregnancy continued.
They were extreme cases.
See page for Kansas stats for abortions at or after 22 weeks gestation.
http://www.kdheks.gov/hci/abortion_sum/08itop1.pdf