Neonatal Anemia After In-Utero Exposure to Natalizumab: A Real-World Retrospective Cohort Study

Background Natalizumab is a monoclonal antibody increasingly used in patients of reproductive age with multiple sclerosis (MS). Although available data suggest early pregnancy exposure is safe, concerns remain regarding potential neonatal hematologic complications. Objective To evaluate whether in-utero exposure to Natalizumab is associated with an increased risk of neonatal anemia in a cohort of pregnant patients with MS. Materials and methods We conducted a retrospective cohort study including all pregnant patients with MS who were monitored throughout their pregnancy between January 2018 and January 2023. Pregnancies were classified as exposed or unexposed to Natalizumab. The primary outcome was neonatal anemia. Secondary outcomes included mean hemoglobin concentration, platelets and leukocyte counts (when available) and neonatal hospitalization for reasons other than anemia. A predefined subgroup analysis assessed third-trimester exposure. Results We included 62 pregnancies, 18 of which had been exposed to Natalizumab. No significant differences were observed for neonatal anemia, except for a lower mean hemoglobin concentration in exposed neonates (14.5 vs 15.9 g/dL, p=0.03), without clinical impact. No other neonatal hematologic abnormality was observed although interpretation is limited by missing data in the non-exposed population. No exposed neonate required transfusion or hospitalization for anemia. Subgroup analysis confirmed these findings. Conclusion Natalizumab exposure during pregnancy, including third trimester, was not associated with an increased risk of clinically significant neonatal anemia in our unicentric retrospective cohort. These findings provide reassuring safety data, although larger prospective studies are required to help therapeutic management during pregnancy.