Mo. Rev. Stat. § 81.240: Change not to be made, when (cities of 20,000 and less than 250,000).

Territory shall not be annexed to any such city within four months next preceding any municipal election, nor shall there be a redistricting or division of the city into wards, or change of boundaries of any ward or wards, or creation of any new ward or wards, within two months next preceding any municipal election.