Abstinence? Naiveity doesn't suit you. The majority of these operations in the third trimester are performed because:
The fetus is dead.
The fetus is alive, but continued pregnancy would place the woman's life in severe danger.
The fetus is alive, but continued pregnancy would grievously damage the woman's health and/or disable her.
The fetus is so malformed that it can never gain consciousness and will die shortly after birth. Many which fall into this category have developed a very severe form of hydrocephalus.
In addition, some physicians violate their state Medical Association's regulations and perform elective D&X procedures - primarily on women who are suicidally depressed.
Overhype by the right to lifers severly overstated the number of these operations performed annually, the best numbers are around 2 - 5 thousand a year.