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Vitamin D influences immune development, but its role in childhood asthma remains unclear. Findings from studies examining vitamin D exposure from pregnancy into early childhood in relation to childhood asthma have been inconsistent, and few have assessed exposure longitudinally. We examined vitamin D status from pregnancy through early childhood to identify the critical window for physician-diagnosed asthma.

Among 205 children from the Chiba High-Risk Birth Cohort for Allergy, we analyzed serum 25-hydroxyvitamin D (25(OH)D) measured at five time points: maternal blood at 36 weeks' gestation, cord blood at birth, and child blood at 1, 2, and 5 years. Asthma was diagnosed at age 5 years using national guideline criteria and classified as no asthma, suspected asthma, or asthma. Multinomial logistic regression models assessed associations adjusted for maternal asthma, prenatal smoking exposure, and birth season.

Maternal and cord blood 25(OH)D concentrations were low (median: 12.0 ng/mL in maternal blood and 6.0 ng/mL in cord blood), whereas postnatal concentrations ranged from 21.0 to 23.4 ng/mL in ages 1-5 years. Cord blood 25(OH)D levels showed a marginal inverse association with asthma (adjusted odds ratio per 1 ng/mL, 0.801; 95% confidence interval: 0.635-1.010). Maternal and postnatal 25(OH)D concentrations showed no clear associations with asthma risk, although estimates showed a similar inverse trend.

Among measurements from late pregnancy through early childhood, cord blood 25(OH)D showed the strongest association with asthma risk at age 5 years, suggesting that fetal vitamin D exposure is a particularly relevant window for asthma development.