The 21-year-old has played all seven games for the League Two club since his arrival at the start of October.
He scored his first career goal in a 3-2 home win against Leyton Orient and has helped the club climb from 21st to 15th in the division.
Hancox has made a total of 40 appearances for Birmingham since making his first-team debut in October 2012.
Defender Mitch Hancox has extended his loan spell at Crawley Town from Birmingham City into a second month.