In 2003, Hejnová competed at the 2003 World Youth Championships in Athletics at Sherbrooke, Canada and won the gold medal in the 400-metre hurdles with a time of 57.54. A year later, she returned at Grosseto, Italy at the 2004 World Junior Championships. This time she won the silver with a time of 57.44 in the 400 m hurdles, defeated by the Russian, Ekaterina Kostetskaya, who won gold by a large margin at 55.55 seconds.