ObjectiveTo assess the effects of autoantibodies on the course of pregnancy and fetal growth.MethodsOne thousand one hundred seventy-nine healthy women with singleton gestations were screened in early pregnancy for seven kinds of autoantibodies: antithyroid microsomal antibody, antithyroglobulin antibody, two kinds of rheumatoid factor, antinuclear antibody, anti-DNA antibody, and antimitochondrial antibody.ResultsIn 228 cases (19.3%), at least one autoantibody was found; however, overlap of autoantibodies in the same individual was unexpectedly rare, and only two cases were positive for as many as four autoantibodies. A significantly higher rate of spontaneous abortion was observed in antibody-positive subjects, especially those with antithyroid microsomal (10.4%) or antinuclear antibodies (16.0%), compared with all antibody-negative subjects (5.5%). There were no significant differences in any outcome assessed among subjects positive for antithyroglobulin antibody, anti-DNA antibody, or antimitochondrial antibody compared with all antibody-negative subjects. None of the seven autoantibodies affected the rates of preterm delivery, stillbirth, pregnancy-induced hypertension, malformation, or gender ratio.ConclusionAntithyroid microsomal antibody and antinuclear antibody are the only autoantibodies that increase the abortion rate.