Background: Relapsing-remitting multiple sclerosis (RRMS) is frequently diagnosed in women of reproductive age. Because the use of disease-modifying therapies (DMTs) early in the disease course is increasing, it is important to evaluate the safety of DMTs in pregnant women and their developing fetuses. Alemtuzumab, approved for the treatment of relapsing forms of MS, is administered as 2 courses of 12 mg/day on 5 consecutive days at baseline and on 3 consecutive days 12 months later. Alemtuzumab is eliminated from the body within approximately 30 days after administration; it is recommended that women of childbearing potential use effective contraception during and for 4 months after treatment. Here, we report pregnancy outcomes in alemtuzumab-treated women from the phase 2 and 3 clinical development program over 16 years., Methods: We followed 972 women who had alemtuzumab in phase 2 (CAMMS223 [NCT00050778]) and phase 3 (CARE-MS I [NCT00530348], CARE-MS II [NCT00548405]) studies, and/or in 2 consecutive extension studies (NCT00930553; NCT02255656 [TOPAZ]). In the extension studies, patients could receive additional alemtuzumab (12 mg/day on 3 days; ≥12 months apart) as needed for disease activity. All women who received alemtuzumab in the clinical development program were included. Pregnant or lactating patients were followed up for safety., Results: As of November 26, 2018, 264 pregnancies occurred in 160 alemtuzumab-treated women, with a mean age at conception of 32.6 years, and mean time from last alemtuzumab dose to conception of 35.9 months. Of the 264 pregnancies, 233 (88%) were completed, 11 (4%) were ongoing, and 20 (8%) had unknown outcomes; 16 (6%) conceptions occurred within 4 months, and 5 conceptions within 1 month of the last alemtuzumab dose. Of the 233 completed pregnancies with known outcomes, there were 155 (67%) live births with no congenital abnormalities or birth defects, 52 (22%) spontaneous abortions, 25 (11%) elective abortions, and 1 (0.4%) stillbirth. Maternal age was associated with an increased risk of spontaneous abortion in alemtuzumab-treated patients (<35 years: 15%; ≥35 years: 37%; relative risk [RR], 2.46 [95% CI: 1.53-3.95], p=0.0002). Risk of spontaneous abortion was not increased in patients becoming pregnant ≤4 months versus >4 months since alemtuzumab exposure (19% vs 23%; RR, 1.08 [95% CI: 0.41-2.85], p=0.88). Autoimmune thyroid adverse events did not increase risk for spontaneous abortion (patients with vs without thyroid adverse events, 23.7% vs 21.3%; RR, 1.11 [95% CI: 0.69-1.80], p=0.75). Annualized relapse rate was 0.10 and 0.12 in the 2 years prior to pregnancy (post alemtuzumab), and was 0.22, 0.12, and 0.12 in each of the first 3 years postpartum, respectively., Conclusion: Normal live births were the most common outcome in women exposed to alemtuzumab 12 mg or 24 mg in clinical studies. Spontaneous abortion rate in alemtuzumab-treated patients was comparable with rates in the general population and treatment-naive MS patients, and was not increased in women with pregnancy onset within 4 months of alemtuzumab exposure. There was a minimal increase in postpartum relapses., Competing Interests: Declaration of Competing Interest The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article. JO has received consulting and/or speaking fees from Biogen Idec, Celgene, EMD Serono, Sanofi, Novartis, and Roche, and grant/research support from Biogen, Roche, and Sanofi. AA has received consulting and/or speaking fees from Biogen, Roche, and Sanofi, and grant/research support from Bayer Schering Pharma, Biogen Idec, Merck Serono, Roche, and Sanofi. EGC has received honoraria for advisory boards and/or speaking from Almirall, Biogen, Genzyme, Merck, Novartis, Roche, and Teva, and unrestricted research grants from Genzyme and Novartis. CC was compensated for serving as editor, associate editor, or member of an editorial advisory board for Birth Defects Research Part A: Clinical and Molecular Teratology, and received grant/research support from AAAAI/VAMPS, AbbVie Laboratories, Amgen, Apotex, AstraZeneca, Barr Laboratories, Bristol-Myers Squibb, California Department of Public Health, CDC, Celgene, Hoffmann-La Roche, Genentech, GlaxoSmithKline, HRSA, IHC, Janssen Biotech, NIAAA, NIH/HIGM, Pfizer, Regeneron, Sanofi, Segirus, the State of California, and UCB Pharma. JD reports receiving speaking and consulting fees from Biogen, Celgene, EMD Serono, Genentech, Genzyme, Novartis, and Teva. VD has received consulting and/or speaking fees from Biogen, EMD Serono, Sanofi, Novartis, Roche, and Teva. KH received speaker honoraria and research support from Almirall, Bayer, Biogen, Merck, Novartis, Sanofi, and Teva. GJH has received research support from Adamas, Biogen, EMD Serono, Hoffman-La Roche, Mallinckrodt, MedImmune, Novartis, and Sanofi, and consulting fees from Biogen, Celgene, Novartis, and Sanofi. PM has received consulting and speaker fees from Biogen, Novartis, Sanofi, and Teva. MM has received speaking fees from Genzyme and fees for participating in advisory boards for Celgene. DR has received consulting fees from Bayer-Schering Pharma, Biogen, MedDay, Merck Serono, Novartis, Roche, Sanofi, and Teva Neuroscience, and research support from Actelion, Biogen, GW Pharma, Merck Serono, Mitsubishi, MedDay, Novartis, Sanofi, Teva Neuroscience, and TG Therapeutics. JRS and LS have nothing to disclose. RFS has received fees for advisory boards and consulting from Biogen and Sanofi, and lecture fees from Biogen, Merck, Novartis, and Roche. SGV reports receiving consulting and speaking fees from Sanofi, speaking fees from Genentech, consulting fees from EMD Serono, and fees as a study monitor for Millennium Pharmaceuticals, Inc. LC, ND, and CM are employees of Sanofi. DASC has received consulting fees and grant/research support from Sanofi, and lecture fees from Bayer-Schering Pharma and Sanofi., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)