Effects of birth months on rheumatic diseases in South Korea: a nationwide case-control study.

Objectives: Birth month/season impacts the development of certain diseases. However, the effect of birth month/season on the development of rheumatic diseases has not been thoroughly investigated. Thus, the objective of this study was to determine whether birth month/season might affect the development of rheumatic diseases.

Methods: Birth month patterns of patients with various rheumatic diseases were compared with those of the general population. The dataset included 17,247,458 individuals from the health insurance review and assessment service database of Korea.

Results: Among 24 rheumatic diseases, the development of Crohn's disease, ulcerative colitis (UC), rheumatoid arthritis, Sjögren's syndrome, polymyalgia rheumatica, ankylosing spondylitis (AS), gout, and fibromyalgia (FM) was significantly associated with birth month/season. UC and AS were more prevalent in individuals born in February/winter. On the contrary, those who were born in June or July/summer were at a higher risk of gout and FM.

Conclusions: Seasonal variations in infectious agents, sun exposure, and food ingestion during gestation or early infancy seem to explain the association between birth month/season and development of rheumatic diseases.