Eighteen states prohibit abortion after a certain number of weeks' gestation (usually 22 weeks from the last menstrual period).[25] The U.S. Supreme Court held in Webster v. Reproductive Health Services that a statute may create "a presumption of viability" after a certain number of weeks, in which case the physician must be given an opportunity to rebut the presumption by performing tests.[27] Because this provision is not explicitly written into these state laws, as it was in the Missouri law examined in Webster, pro-choice organizations believe that such a state law is unconstitutional, but only "to the extent that it prohibits pre-viability abortions".[28]