Tysabri (natalizumab) is a humanized IgG class monoclonal antibody that will cross the placenta just like most other antibodies. Doses much higher than those used to treat MS may cause adverse effects on an unborn animal but these problems are not readily apparent when lower doses are given to lab animals. The situation in humans is totally unknown except for small case series reporting accidental pregnancy outcomes in woman taking tysabri for their MS. No obvious fetal abnormalities or poor pregnancy outcomes have yet been linked to tysabri in these small case series but it is too early to be sure. This is why Tysabri remains pregnancy category C.
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