When FoxO1 protein expression was assessed using a four‐tiered grading system (0, 1+, 2+, 3+), the cases with acute chorioamnionitis showed a significantly decreased FoxO1 immunoreactivity in the medullary thymus than did those without acute chorioamnionitis did (P = 0.015). This evidence concerns the gene FOXO1 and chorioamnionitis.