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Background: Interferon-beta (IFN-beta) is a commonly used treatment for multiple sclerosis (MS). Current guidelines recommend cessation of treatment during pregnancy, however the results of past studies on the safety of prenatal exposure to IFN-beta have been conflicting. A large scale study of a population of MS women is therefore warranted., Objectives: To assess whether, among those born to women with MS, infants prenatally exposed to IFN-beta show evidence of smaller size at birth relative to infants which were not prenatally exposed to any MS disease modifying drugs., Methods: Swedish and Finnish register data was used. Births to women with MS in Sweden and Finland between 2005-2014 for which a birth measurement for weight, height, and head circumference was available were included. The exposure window was from 6 months prior to LMP to the end of pregnancy., Results: In Sweden, 411 pregnancies were identified as exposed to IFN-beta during the exposure window, and 835 pregnancies were counted as unexposed to any MS DMD. The corresponding numbers for Finland were 232 and 331 respectively. Infants prenatally exposed to interferon-beta were on average 28 grams heavier (p = 0.17), 0.01 cm longer (p = 0.95), and had head circumferences 0.14 cm larger (p = 0.13) in Sweden. In Finland, infants were 50 grams lighter (p = 0.27), 0.02 cm shorter (p = 0.92) and had head circumferences 0.22 cm smaller (p = 0.15) relative to those unexposed., Conclusions: This study provides evidence that exposure to IFN-beta during pregnancy does not influence birth weight, length, or head circumference., Competing Interests: I have read the journal's policy and the authors of this manuscript have the following competing interests: This study was funded by Bayer AG, Biogen Netherlands B.V., Merck KGaA, and Novartis Europharm Limited. SB reports no personal conflict of interest, but works for CPE which receives funding from third parties including pharmaceutical companies. PV reports no personal conflict of interest, but works for EPID Research which is a contract research organization and thus its employees have been and currently are working in collaboration with several pharmaceutical companies. YG is an employee of Novartis Pharma AG. MS is an employee of Merck KGaA. CP is an employee of Biogen. KSW is an employee of Bayer AG. JH has received honoraria for serving on advisory boards or steering committees for Biogen, Merck, Sanofi-Genzyme and Novartis and speaker’s fees from Biogen, Novartis, Merck-Serono, Bayer-Schering, Teva and Sanofi-Genzyme. He has served as P.I. for projects, or received unrestricted research support from, BiogenIdec, Merck-Serono, TEVA, Sanofi-Genzyme and Bayer-Schering. MA reports no conflict of interest. AM reports no conflict of interest. KMM has received unrestricted research grants to his institution and/or scientific advisory board or speakers honoraria from Almirall, Biogen, Genzyme, Merck, Novartis, Roche and Teva; and has participated in clinical trials organized by Biogen, Merck, Novartis, and Roche. PK works for EPID Research which is a contract research organization and thus its employees have been and currently are working in collaboration with several pharmaceutical companies. SM has received funding from AstraZeneca, and funding for MS related research from F. Hoffman-La Roche AG, and Novartis International AG. ShB reports no personal conflict of interest, but works for CPE which receives funding from third parties including pharmaceutical companies. There are no patents, products in development or marketed products to declare. This does not alter our adherence to all the PLOS ONE policies on sharing data and materials.