Missouri Revised Statutes Title XXXI. Trusts and Estates of Decedents and Persons Under Disability § 459.025. Declaration operative, when

The directions of a declarant able to make treatment decisions shall at all times supersede the declaration.  The declaration shall be given operative effect only if the declarant's condition is determined to be terminal and the declarant is not able to make treatment decisions.  Such determinations shall be recorded in the declarant's medical record.  A physician, health care professional or facility or other person shall not act contrary to the declarant's expressed intent to withhold or withdraw death-prolonging procedures without serious reason therefor consistent with the best interest of the declarant.  Such reason shall be recorded in the declarant's medical record.  The declaration to withdraw or withhold treatment by a patient diagnosed as pregnant by the attending physician shall have no effect during the course of the declarant's pregnancy.