"A “State has a legitimate interest in seeing to it that abortion . . . is performed under circumstances that insure maximum safety for the patient.” Roe v. Wade, 410 U. S. 113 . But “a statute which, while furthering [a] valid state interest, has the effect of placing a substantial obstacle in the path of a woman’s choice cannot be considered a permissible means of serving its legitimate ends,” Planned Parenthood of Southeastern Pa. v. Casey, 505 U. S. 833 (plurality opinion), and “[u]nnecessary health regulations that have the purpose or effect of presenting a substantial obstacle to a woman seeking an abortion impose an undue burden on the right,” id., at 878."