Objective: Fetal microchimerism may have a role in development of autoimmune thyroid disorders. Using parity as a surrogate for increasing fetal cell exposure, we analyzed its association with thyroid peroxidase antibody levels. Study Design: Secondary analysis of serum thyroid analytes determined in 17,298 women from a population-based prospective study between 2001 and 2003. Sera were assayed for thyrotropin, free thyroxine, and antithyroid peroxidase antibodies. We analyzed the relationship between thyroid peroxidase antibodies and increasing parity. Results: The incidence of abnormally elevated thyroid peroxidase antibody levels (>50 IU/mL) increased with advancing parity, but was not significant after adjustment for maternal characteristics. However, at higher thyroid peroxidase antibody levels (>500 IU/mL), a significant relationship with advancing parity persisted after adjustments (P =.002). Conclusion: Advancing parity is associated with an increased risk for high serum concentrations of antithyroid peroxidase antibodies. This suggests fetal microchimerism may play a role in development of autoimmune thyroid disorders. © 2011 Mosby, Inc.