Requirements for Consent to Surgical or Medical Treatment - Mo. Rev. Stat. § 431.061

The statute above addresses who may provide consent for medical treatment and under what circumstances. 

Under the statute, any competent adult, age eighteen or older, may provide consent either orally or otherwise, to any lawful medical procedure.  However, minors, persons under age 18, may only provide consent for treatment under the following circumstances:

• The minor has been lawfully married
• The minor is the parent or legal custodian of a child, in which case the minor can also provide consent for that child
• The minor is seeking care for pregnancy (excluding abortions), venereal disease or drug or substance abuse.

In all other situations, any parent may provide consent for a minor child in that parent’s legal custody.  Also, in cases of emergencies any adult standing in loco parentis may provide consent for his or her minor charge.  During the absence of a parent, an adult sibling or grandparent of a minor child may provide consent.  Consent for a minor provided by one person who is authorized will be sufficient regardless of the objection of other persons also authorized to provide consent for that minor.