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Background While anti-β2 glycoprotein 1 (anti-β2GP1) antibody positivity is included in the diagnostic criteria for antiphospholipid syndrome (APS), the association between anti-β2GP1 and the obstetrical complications of APS has been inconsistently reported and remains unclear. Objective We completed a case-control study nested within the Canadian Ottawa and Kingston (OaK) Birth Cohort to evaluate the association between anti-β2GP1 antibody positivity and placenta-mediated pregnancy complications. Study Design Five hundred cases were randomly selected among pregnant women who experienced any of the following independently adjudicated placenta-mediated pregnancy complications: preeclampsia, placental abruption, late pregnancy loss (≥ 12 weeks' gestation), and birth of a small-for-gestational age (SGA) infant < 10th percentile. Five hundred pregnant women without any placenta-mediated pregnancy complications were selected as controls. Stored blood samples were analyzed for the presence of anti-β2GP1 antibodies by enzyme-linked immunosorbent assay. Results Anti-β2GP1 immunoglobulin G (IgG) and/or immunoglobulin M (IgM) antibodies in titers ≥ 20 G/M units (> 99th percentile) were present in 24 of 497 (4.8%) of controls and 33 of 503 (6.6%) of cases. There was no significant difference between cases and controls for the composite outcome of any placenta-mediated pregnancy complications (odds ratio, 1.38, 95% confidence interval [CI], 0.8-2.37, p = 0.25). Conclusion Our results call into question the association between anti-β2GP1 antibodies and placenta-mediated pregnancy complications, with further research needed. [ABSTRACT FROM AUTHOR]