On 10 September, the O2 Arena in London will be the site of a fight for the undisputed title.
Marshall holds the WBO title, while American Shields holds five different titles.
Marshall is 10 knockouts better than Shields.
The first all-female card to be hosted in the United Kingdom will feature a super-featherweight unification bout.
American Baumgardner holds the WBC belt and is a champion in three different weight classes.
The fight between Marshall and Shields has been going on for a decade.
This is the biggest rivalry ever to come from women's boxing, according to Boxxer promoter.
The fight was delayed in May because of an injury to Marshall and will now take place in London rather than the original location.
Shields' only defeat in her entire boxing career was against Marshall when they were both preparing for the 2012 Olympics, a Games at which Shields would win a gold medal and Marshall would bow out.
They sparred in the ring, but never fought again. The loss to Marshall when she was 17 has always been a sore spot for Shields.
The first boxer to hold two undisputed titles in two weight divisions at the same time was an American who won world titles at multiple weights.
She won gold at the 2012 Olympics and is returning to fight in London.
Marshall's amateur career never reached the heights it was supposed to, but his pro career has gone from strength to strength in the last two years.
She is considered to be one of the biggest punchers in women's boxing with 10 knockouts and will be looking to get her ninth in a row against Shields.
There is a chance that the winner of the Mayer-Baumgardner contest could set up a fight against the South Korean title-holder.
The 2016 Rio Olympian won the WBO belt in October 2020. In eight months, she added the Ring Magazine straps to her collection.
Baumgardner, who has won 13 and lost one, has made one defence of the title he won last year.
There's more to come.
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