[8] Baumgardner suffered her first loss as a professional in her first WBC International title defense, at the hands of Christina Linardatou on July 28, 2018.

[12] In October 2021, it was announced that Baumgardner would face WBC and IBO female super featherweight champion Terri Harper on 13 November 2021, Utilita Arena Sheffield.

During the fight she dominated the second round and then caught Harper with a vicious right hand in the fourth, resulting in her going unbalanced with her whole body stiffened, which caused the referee to jump in to stop the bout.

[14][2][1] The referee was later praised by boxing pundits and fans for stepping in to stop the fight as Baumgardner had her hand in the air ready to pounce on Harper.

[19] In January 2021, it was also adjudged as Female Knockout of the Year by BoxingScene.com beating three others including Zulina Muñoz and Daniela Romina Bermúdez.

The fight was televised live on DAZN as part of the undercard of Conor Benn's WBA continental title defense bout against Chris van Heerden.

[23][22] Baumgardner successfully retained her WBC and IBO titles via unanimous decision (UD), dominiating over ten rounds, with all three judges' scorecards reading 100–90.

[25] Boxing analysts and former world champions Timothy Bradley and Andre Ward noted Baumgardner's fast start, athletic style, and strong finish that seemingly flustered Mayer.

[25] On November 15, 2022, the WBA ordered their junior lightweight champion Choi Hyun-mi to face Baumgardner in a title unification bout.