The Blues defender, who has completed one month on loan at The Swans, will remain with the League One side for a further month although Blues have a recall option on the player.
"We have extended Marcos's loan spell," Swans boss Kenny Jackett told his club's official website.
"Whether it will go beyond a second month I don't know at this stage.
"Birmingham manager Steve Bruce wants to see how his side comes through the busy Christmas and New Year period first with regards to injuries.
"For a 20-year-old he is proving to be a very good player."