To determine (1) the number of pregnancies in women with multiple sclerosis (MS) exposed to Interferon beta (IFN-β) in later-stage pregnancy (i.e. the 2^nd and 3^rd trimesters) in Finland and Sweden; (2) whether the number of pregnancies available in Finland and Sweden is adequate for a cohort study to assess the outcomes of IFN-β exposure in later-stage pregnancy.

Previous studies showed no increased risk of IFN-β use before or early in pregnancy among women with MS. However, knowledge about the utilisation and safety of IFN-β in later-stage pregnancy remains limited.

This observational drug utilisation study (DUS), using national register data in Finland (1996–2022) and Sweden (2005–2022), will report the number of pregnancies in women with MS (1) exposed to IFN-β only in later-stage pregnancy and (2) unexposed to any MS disease-modifying drugs. The 95% confidence intervals (CIs) of detectable relative risks (RR) for pre-defined adverse pregnancy outcomes are calculated for study size simulation. Based on the available background prevalences among the unexposed (P_background) and assuming 4% of all pregnancies are exposed, 100 exposed pregnancies are expected to be sufficient to detect all anomalies, spontaneous abortions, elective terminations (RR=2.00, 95%CI 1.40–2.89; P_background 12.0%); preterm birth (RR=2.00, 95%CI 1.28–3.18; P_background 8.0%); low birth weight (RR=2.00, 95%CI 1.03–4.05; P_background 4.0%); major congenital anomalies (RR=2.50, 95%CI 1.32–4.92; P_background 3.5%); and small for gestational age (RR=2.50, 95%CI 1.08–6.29; P_background 2.0%).

100 exposed later-stage pregnancies are considered the minimum number for a cohort study on the adverse pregnancy outcomes. First results of this DUS will be reported in 2024. In case of inadequate number of pregnancies, the DUS will be extended for 2 years.

This DUS (EUPAS38736) will evaluate whether the accrued number of exposed pregnancies is adequate for a cohort study to evaluate the safety of IFN-β exposure in later-stage pregnancy among women with MS.