Girl I’m seeing has no eyebrows?

I didn’t notice because she’d draw them in but she has extremely thin almost invisible eyebrows (like from a distance it looks like she has no brows) and today was the first time I saw her with her natural brows. Is this a genetic thing? I’m curious because I read it could be due to chemotherapy or hormone problems. No hate and I still find her attractive but I wanna know what could possibly cause thin almost invisible brows and I don’t wanna ask her in fear of scaring her away or making her feel bad.