Pregnancy complications and birth outcomes in women with antiphospholipid antibodies: a Danish population-based cohort study.

OBJECTIVES: To provide the first population-based estimates of pregnancy complications, adverse birth outcomes, and venous thromboembolism (VTE) among clinically selected women undergoing antiphospholipid antibody (aPL) testing, and to examine how risks vary by aPL status. METHODS: We conducted a population-based cohort study in the Central Denmark Region (2012-2023), including pregnancies reaching GA10 in women with at least one aPL measurement before pregnancy. Pregnancies were classified by aPL status and APS-related outcomes were assessed according to the 2023 ACR/EULAR criteria. Generalized estimating equations were used to estimate risk ratios (RR), risk differences, and mean differences with 95% confidence intervals (CI). RESULTS: Among 2,961 pregnancies, 137 (4.6%) were aPL-positive. Positive aPL was not associated with spontaneous abortion between GA10 + 0 and 15 + 6 or unexplained fetal death before GA34 + 0. However, aPL-positive pregnancies had higher risks of intrauterine growth restriction <GA34 (2.9% vs 1.0%; RR 3.00 [1.00-8.6]), VTE (1.5% vs 0.3%; RR 5.4 [1.14-26.0]), and late preterm birth (14% vs 6.4%; RR 2.28 [1.41-3.66]), and greater negative deviation from expected birth weight (adjusted mean difference -3.4% [-6.1--0.73]). Substantially fewer APS-related events than expected resulted in imprecise estimates. aPL-negative women also had elevated risks compared with the general population. CONCLUSION: In this population-based cohort of clinically tested women, absolute risks associated with aPL positivity were lower than those reported in APS referral cohorts. These findings suggest that the prognostic implications of aPL positivity depend strongly on the clinical context of testing and should inform clinical risk assessment and counseling.