Background <p>Formation of compact plasma fibrin clots with impaired lysability was observed in obstetric antiphospholipid syndrome (OAPS). Whether unfavorably modified fibrin clot properties contribute to future obstetric complications in OAPS remains unknown.</p> Objectives <p>To investigate whether plasma fibrin clot phenotype is associated with pregnancy outcomes in OAPS.</p> Methods <p>In patients with OAPS and controls matched for age, body mass index (BMI), and type of obstetric complications (both <i>n</i> = 62) fibrin clot permeability (K_s) and clot lysis time (CLT), thrombin activatable fibrinolysis inhibitor (TAFI), plasminogen, α₂-antiplasmin and endogenous thrombin potential (ETP) were determined. Pregnancy and live births were recorded during a median follow-up period of 54 months.</p> Results <p>At baseline, obstetric complication rates did not differ between the groups. During follow-up, pregnancy was confirmed in 72.6% of women with OAPS and 71% of controls, with identical proportions of live births (51.6%; <i>n</i> = 32). Among OAPS, women with live births compared to women without live births had higher baseline K_s (+ 18.2%, <i>p</i> &lt; 0.0001) without any difference in CLT. Baseline K_s ≤6.1 × 10^− 9cm² discriminated lower live birth rates in OAPS. Pregnant women with OAPS had higher ETP (+ 5.1%) and TAFI (+ 14.4%), along with lower plasminogen (-6%) and α₂-antiplasmin (-6.7%) compared with pregnant controls. Women with live births showed slightly higher ETP (+ 1.5%) and increased TAFI level (+ 12.0%) with decreased plasminogen (-10%), and α₂-antiplasmin (-10%) compared to the controls with live births (all <i>p</i> &lt; 0.05).</p> Conclusion <p>Prothrombotic fibrin clot properties are observed in women with OAPS with lower live birth rate, suggesting a potential role of fibrin-related mechanisms in this manifestation.</p>