Background/Objectives: ACR/EULAR has recently developed new classification criteria for antiphospholipid syndrome (APS). The present study aims to analyze the impact of these new 2023 ACR/EULAR classification criteria in a cohort of pregnant women with primary APS. Methods: Retrospective cohort study of 93 consecutive pregnant women attending the Autoimmune Diseases Pregnancy Clinic, a multidisciplinary unit of a tertiary care teaching hospital, between 2005 and 2023. All of them fulfilled the Sydney classification criteria for APS. Women diagnosed with rheumatic autoimmune diseases other than APS were excluded. Results: Twenty-four out of ninety-three patients (25.8%) met the 2023 ACR/EULAR criteria for APS. Patients who met the new classification criteria were very similar to those who did not, except for being younger (p < 0.001), and had a lower number of clinical pregnancies (p = 0.004). The obstetric domain was clearly underrepresented in women who fulfilled the 2023 ACR/EULAR criteria (p < 0.001). Patients meeting the new classification criteria were primarily characterized by preterm births before 34 weeks due to severe placentation disorders (p = 0.004). Women with early and late fetal loss were significantly underrepresented (p < 0.0001 and 0.03, respectively). Nearly half of these patients had thrombocytopenia (p < 0.001). Serologically, these patients showed a higher frequency of persistent lupus anticoagulant (p = 0.02) and a lower frequency of IgM isotype antiphospholipid antibodies (p = 0.05). Conclusions: Almost three-quarters of the patients included in the study did not meet the 2023 ACR/EULAR criteria. Most patients who could not be classified according to these new classification criteria were those with early and/or late fetal deaths, as well as patients carrying only IgM aCL/AB2GPI antibodies. The high specificity of the 2023 ACR/EULAR criteria, restricted to severe placentation disorders, may leave the majority of patients with obstetric APS out of the new classification criteria.
