"In the almost 30 years since Roe, this Court has never described the various methods of aborting a second- or third-trimester fetus. From reading the majority’s sanitized description, one would think that this case involves state regulation of a widely accepted routine medical procedure. Nothing could be further from the truth. The most widely used method of abortion during this stage of pregnancy is so gruesome that its use can be traumatic even for the physicians and medical staff who perform it. See App. 656 (testimony of Dr. Boehm); W. Hern, Abortion Practice 134 (1990). And the particular procedure at issue in this case, “partial birth abortion,” so closely borders on infanticide that 30 States have attempted to ban it. I will begin with a discussion of the methods of abortion available to women late in their pregnancies before addressing the statutory and constitutional questions involved."