There were complications during Fleur’s birth, and she was technically stillborn. Healers were able to bring her back, but she was ‘dead’ for over 3 minutes.
Fleur’s parents were naturally concerned about Fleur but saw no complications as she grew up. Naturally, they were reticent to try again and didn’t try for another child until 8 years after Fleur was born.
It became apparent later that there were complications, after Fleur transformed for the first time and her wings were black instead of the usual white of the European veela.
After her first transformation she could also see what she soon realized was death hanging around people. It was especially heavy around older or ill people. The one time she followed her parents to a murder trial she saw death hanging around the accused, yet it was somehow different from the elderly or ill.
Fleur continued with her life her strange abilities not affecting her life much, especially living outside of an enclave as she rarely transformed. However, when she went to Hogwarts, she was shocked to see the amount of death hanging around the Boy-Who-Lived.