Anticardiolipin antibodies (ACLA) have recently been associated with adverse fetal outcome. The prevalence of elevated ACLA has not been studied in unselected pregnant women, however. Twelve hundred unselected pregnant women were screened for IgG ACLA using an assay standardized by the first international workshop on ACLA. Fifteen (1.25%) were positive for IgG ACLA (greater than 3 SD above the mean) but only 0.5% had moderate to high levels of IgG ACLA (greater than 5 SD above the mean). Low levels of IgG ACLA were not associated with increased risk of fetal loss; however, 50% of women with moderate to high levels of antibody had fetal wastage. These findings further support the association of significantly elevated levels of IgG ACLA with fetal loss.