To avoid legal problems. Executing them with a lethal drug before they start the procedure:

In response to the Supreme Court decision upholding the Partial-Birth Abortion Ban Act, many abortion providers in Boston and around the country have adopted a defensive tactic. To avoid any chance of partially delivering a live fetus, they are injecting fetuses with lethal drugs before procedures.
That clinical shift in late-term abortions goes deeply against the grain, some doctors say: It poses a slight risk to the woman and offers her no medical benefit.
“We do not believe that our patients should take a risk for which the only clear benefit is a legal one to the physician,” Dr. Philip D. Darney, chief of obstetrics at San Francisco General Hospital, wrote in e-mail. He has chosen not to use the injections.
[…]
Instead, doctors typically cause the fetus’s death surgically while it is still inside the womb and then remove it.
But now, if the fetus is not dead as it begins to emerge, a provider may be accused of violating the law. So the lethal injections beforehand, carefully documented, are aimed at precluding an accusation and prosecution.

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