Effects of birth order on the prevalence of allergic diseases in Taiwanese children: A 16-year study using two national databases.

Taiwan has experienced a significant decline in fertility rates, from 3.69 in 1970 dropping to 1.5 in 1998, and 0.87 in 2023. Simultaneously, the prevalence of childhood allergic conditions has surged: atopic dermatitis increased from 1.4% in 1974 to 11.3% in 2019, asthma from 1.3% to 20.3%, and allergic rhinitis from 7.8% to 50.6%. This study investigates the relationship between Taiwan's declining birth rate and the rising prevalence of allergic diseases among children. We analyzed data from the Taiwan Birth Registration Database (2004-2014) linked to the National Health Insurance Research Database. Children were categorized by birth order, and adjusted hazard ratios were used using multivariable Cox proportional hazards models. Subgroup analyses confirmed consistency across sex, maternal age, SES, and birth characteristics. National total fertility rate and allergy prevalence data from 1974 to 2023 were analyzed to explore long-term trends. Higher birth order was associated with lower allergic disease risk. Compared to first-borns, second-born and third-or-later-born children had reduced risks of atopic dermatitis (by 21.3% and 37.5%), asthma (25.0% and 26.9%) and allergic rhinitis (16.3% and 29.3%). Nationally, the total fertility rate was strongly inversely correlated with the prevalence of AD (r = -.91), asthma (r = -.88), and AR (r = -.95) (all p < .001). These findings suggest that declining fertility and smaller family size may contribute to increased childhood allergy prevalence, potentially through reduced microbial exposure in early life.