Inclusion Criteria:  Woman is willing to breastfeed for at least 60 days after the first post-partum ocrelizumab infusion (this decision is to be taken prior to and independent from study participation) Woman is willing to provide breastmilk samples Woman has a diagnosis of MS or CIS (in line with the locally approved indications) Woman has delivered a term singleton infant (≥37 weeks gestation) Infant is between 2-24 weeks of life For women who received commercial ocrelizumab (OCREVUS) before enrolment: documentation that last exposure to ocrelizumab occurred more than 3 months before the last menstrual period (LMP) and was given at the approved dose of 2 x 300 mg or 1 x 600 mg Woman agrees to use acceptable contraceptive methods during the study  Exclusion Criteria related to the Mother:  Received last dose of ocrelizumab <3 months before the LMP or during pregnancy Active infections (may be included once the infection is treated and is resolved; women with bilateral mastitis infection should not have samples collected until the infection is completely resolved) Prior or current history of primary or secondary immunodeficiency, or woman in an otherwise severely immunocompromised state Known active malignancies, or being actively monitored for recurrence of malignancy History of breast implants, breast augmentation, breast reduction surgery or mastectomy Prior or current history of chronic alcohol abuse or drug abuse Any medical, obstetrical or psychiatric condition that, in the opinion of the Investigator, would compromise the woman's ability to participate in this study Positive screening tests for hepatitis B Treatment with a DMT for CIS or MS during pregnancy and/or first weeks post-partum, with the exception of formulations of interferon-beta, glatiramer acetate or pulsed corticosteroids Treatment with drugs known to transfer to the breastmilk and with established or potential deleterious effects for the infant Treatment with any investigational agent within 6 months or five half-lives of the investigational drug prior to the LMP  Exclusion Criteria related to the Infant:  >24 weeks of life at the time of the mother's first dose of ocrelizumab Any abnormality that may interfere with breastfeeding or milk absorption Active infection (may be included once the infection resolves) Any other medical condition or abnormality that, in the opinion of the investigator, could compromise the infant's ability to participate in this study At least one documented brief resolved unexplained event (BRUE), as defined by the 2016 Guidelines of the American Academy of Pediatrics