BackgroundPatients with multiple sclerosis (MS) or Crohn’s disease (CD) being treated with natalizumab (Tysabri®, Biogen) who are planning to become pregnant or discover they are pregnant after exposure to natalizumab are currently advised to balance the potential benefits and potential risks of exposure when considering treatment options. This study was undertaken to evaluate pregnancy outcomes of women with MS or CD who were exposed to natalizumab at any time within 3 months prior to conception or during pregnancy. A pregnancy registry was created to better understand the effect of natalizumab exposure on pregnancy outcomes.MethodsThe Tysabri Pregnancy Exposure Registry was a global, observational exposure registration and follow-up study. Evaluations included spontaneous abortions (<22 weeks gestational age), fetal losses (≥22 weeks gestational age), ectopic pregnancies, elective or therapeutic terminations, stillbirths, birth defects, and live births. Birth defects were reviewed and coded in accordance with the Metropolitan Atlanta Congenital Defects Program (MACDP) classification of birth defects.ResultsA total of 369 patients with MS and 7 patients with CD were enrolled prospectively, of whom 355 patients (99.4 %; 349 MS and 6 CD) had known pregnancy outcomes (including 8 twin sets). The spontaneous abortion rate was 9.0 % (n = 32; 95 % confidence interval [C. I.], 6.3–12.5 %). An independent advisory committee review determined the major birth defect rate to be 5.05 % (16 of 316 live births + 1 elective abortion; 95 % C. I., 2.9–8.1 %). The mean gestational age of the live-born infants was 38.3 weeks, and the mean birth weight was 3158.3 g.ConclusionsAlthough the overall rate of birth defects was higher than that observed by the MACDP, these registry outcomes showed no specific pattern of malformations that would suggest a drug effect, and the spontaneous abortion rate was consistent with that of the general population.Trial registrationClinicalTrials.gov NCT00472992 (11 May 2007).