Background: Although patients with multiple sclerosis (MS)are advised to stop interferon (IFN) beta-1a therapy before becoming pregnant, some patients become pregnant while on treatment. Methods: We examined individual patient data from eight clinical trials with IFNβ-1a. Results: Of 3,361 women in the studies, 69 pregnancies were reported, of which 41 were patients receiving (or who had stopped receiving within 2 weeks prior to conception) IFNβ-1a (in utero exposure group), 22 were patients who discontinued IFNβ-1a treatment more than 2 weeks before conception (previous exposure group), and six were patients receiving placebo. The 41 in utero exposure pregnancies resulted in 20 healthy full-term infants, one healthy premature infant, nine induced abortions, eight spontaneous abortions, one fetal death, and one congenital anomaly (hydrocephalus). One patient was lost to follow-up. The 22 previous exposure pregnancies resulted in 20 full-term healthy infants, one healthy premature infant, and one birth-related congenital anomaly (Erb palsy) Conclusions: The majority (21/31) of pregnancies that had the potential to go to full term produced healthy infants. The rate of spontaneous abortion was higher, but not significantly so, in the in utero exposure group compared to general population estimates. Until more exposure data become available, patients remain advised to stop IFNβtherapy before becoming pregnant. Background: Although patients with multiple sclerosis (MS) are advised to stop interferon (IFN) beta-1a therapy before pregnant while on treatment. Some patients became pregnant while on treatment. Methods: We examined individual patient data from eight clinical trials with IFNbeta-1a. Results: Of 3,361 women in the studies, 69 pregnancies were reported, of which 41 were patients receiving (or who had stopped receiving within 2 weeks prior to conception) IFNβ-1a (in utero exposure group), 22 were patients who discontinued IFNβ- 1a treatment more than 2 weeks before conception (previous exposure group), and six were patients receiving placebo. The 41 in utero exposure pregnancies resulted in 20 healthy full-term infants, one healthy premature infant, nine induced abortions, eight spontaneous abortions, one One death was forced to follow-up. The 22 previous exposure pregnancies resulted in 20 full-term healthy infants, one healthy premature in fant, and one birth-related congenital anomaly (Erb palsy) Conclusions: The majority (21/31) of pregnancies that had the potential to go to full term produced healthy infants. The rate of spontaneous abortion was higher, but not significantly so, in the in utero exposure group compared to general population estimates. Until more exposure data become available, patients remain advised to stop IFNβtherapy before becoming pregnant.