OBJECTIVE: To assess fetal risk after pregnancy exposure to natalizumab in women with multiple sclerosis (MS), with a specific focus on spontaneous abortion (SA) and congenital anomalies (CA). METHODS: Data of all pregnancies occurring between 2009 and 2015 in patients with MS treated with natalizumab and referring to 19 participating sites were collected and compared with those of pregnancies in untreated patients and patients treated with injectable immunomodulatory agents. Rates of SA and CA were also compared with those reported in the Italian population. Multivariable logistic and linear regression models were performed. RESULTS: A total of 92 pregnancies were tracked in 83 women. In the multivariable analysis, natalizumab exposure was associated with SA (odds ratio [OR] 3.9, 95% confidence interval [CI] 1.9-8.5, p <0.001). However, the rate of SA (17.4%) was within the estimates for the general population, as well as the rate of major CA (3.7%). Moreover, exposure to natalizumab and interferon-β (IFN-β) was associated with lower length and weight of the babies (p <0.001). CONCLUSION: Our results showed that natalizumab exposure to up 12 weeks of gestation is associated with an increased risk of SA, although within the limits expected in the general population, whereas the risk of CA needs further investigation. Taking into account the high risk of disease reactivation after natalizumab suspension, pregnancy could be planned continuing natalizumab while strictly monitoring conception. CLASSIFICATION OF EVIDENCE: This study provides Class III evidence that in women with MS, natalizumab exposure increases the risk of spontaneous abortion as compared to IFN-β-exposed or untreated patients (OR 3.9, 95% CI 1.9-8.5). © 2018 American Academy of Neurology.