Mo. Rev. Stat. § 474.220: Waiver of right to elect.

The right of election of a surviving spouse hereinbefore given may be waived before or after marriage by a written contract, agreement or waiver signed by the party waiving the right of election, after full disclosure of the nature and extent of the right, if the thing or the promise given to the waiving party is a fair consideration under all the circumstances. This written contract, agreement or waiver may be filed in the same manner as hereinbefore provided for the filing of an election.