It isnt.
"Partial birth abortion" isnt a medical term, nor does the description in the opening post describe any medical procedure. The fetus remains inside the body at all times, but the womb is broken open so that the fetus can be removed (this is where the words "partial birth" come from).
That is impossible.
These kinds of abortions are performed when the fetus is hydrocephalic (that means the head has swollen with fluid and cannot be safely delivered), or the fetus has died, or the fetus is severely malformed. If the fetus is hydrocephalic, then fluid has to be drained (this is where the description of "ramming a sharp instrument into the babies head and sucking it out through a tube" comes from - the description is meant to sound shocking, but the baby cannot pass through the womb unless the fluid is removed).
Adding to that, the fetuses cannot be safely carried to term, nor would they survive outside of the womb. There are only two options, the mother could carry to term (risking her life or the ability to have future pregnancies) or she could abort, there is no choice for adoption. About 95% of partial birth abortions take place before the 20th week, and none occur after 24 weeks.
As for the inhumanity of partial birth abortion, they represent 1% of the total number of abortion in the US, they are only performed on women who would be disabled or die if the fetus were carried to term (contrary to popular belief, there are stict laws regulating this procedure), and most states require fetal anesthetic before the procedure occurs. See
ReligiousTolerance.org -Partial Birth Abortion.