The constitutional interpretation is frequent object of reflection of the legal doctrine contemporary, the occurrence of conflict between effective norms and a magnifying of the importance of principles in the texts constitutional demands a new proposal of methodic in the interpretation of the Constitution. The study of case formulated, the proposal of legal permission for therapeutical anticipation of childbirth in case of fetal anencephaly, its repercussion in the diverse sectors of the Brazilian society. The laical rationality used by educations of Bioethics is essential for the construction of legitimate arguments in the deliberative process. The empirical data of the Instituto Fernandes Figueira prove the specialty of this type of gestation, imposing a new analysis in which the legal knowledge receive the complement theoretical from the medical area. This interconnection of the scientific fields causes one dynamic study of the Law, not being able the understood of the Law as a closed system of norms. The conflict of basic values in the study of case and the provocation of a judicial solution sends to a questioning on the constitutional function of the Brazilian Supreme Federal Court and its paper on the Democratic State of Right.