"Justice O'CONNOR, Justice KENNEDY, and Justice SOUTER concluded in Part IV that an examination of Roe v. Wade, 410 U.S. 113, 93 S.Ct. 705, 35 L.Ed.2d 147, and subsequent cases, reveals a number of guiding principles that should control the assessment of the Pennsylvania statute: (a) To protect the central right recognized by Roe while at the same time accommodating the State's profound interest in potential life, see, id., at 162, 93 S.Ct., at 731, the undue burden standard should be employed. An undue burden exists, and therefore a provision of law is invalid, if its purpose or effect is to place substantial obstacles in the path of a woman seeking an abortion before the fetus attains viability. (b) Roe's rigid trimester framework is rejected. To promote the State's interest in potential life throughout pregnancy, the State may take measures to ensure that the woman's choice is informed. Measures designed to advance this interest should not be invalidated if their purpose is to persuade the woman to choose childbirth over abortion. These measures must not be an undue burden on the right. (c) As with any medical procedure, the State may enact regulations to further the health or safety of a woman seeking an abortion, but may not impose unnecessary health regulations that present a substantial obstacle to a woman seeking an abortion. (d) Adoption of the undue burden standard does not disturb Roe's holding that regardless of whether exceptions are made for particular circumstances, a State may not prohibit any woman from making the ultimate decision to terminate her pregnancy before viability. (e) Roe's holding that "subsequent to viability, the State in promoting its interest in the potentiality of human life may, if it chooses, regulate, and even proscribe, abortion except where it is necessary, in appropriate medical judgment, for the preservation of the life or health of the mother" is also reaffirmed. Id., at 164-165, 93 S.Ct., at 732. Pp. ____."