Most of St. Charles County is part (not all) of Blaine Luetkemeyer's 3rd District.
I live in a part of St. Louis County that has gone back and forth between the 2nd and 1st District, with the last redistricting putting me back in the 2nd.
Based on the last three elections, about 38% of St. Charles County voters are in the 2nd District and 62% are in the 3rd District. Wagner and Luetkemeyer did equally well in 2016, with each receiving about 63% of the vote, and both did even better in 2014.
Wagner does not do as well in St. Louis and Jefferson Counties, but still has always received the majority of votes in each county. In each of her three elections to the house, her overall margin over her Democrat opponent has never been less than 20%.
Based on this history, it would take a huge wave to upset her in 2018.