"Myasthenia may improve, worsen, or remain unchanged during pregnancy and it is not uncommon for the first symptoms of MG to begin during pregnancy or postpartum . . . . abortion is rarely, if ever, needed because of MG and the frequency of spontaneous abortion is not increased."
For more details on MG and pregnancy see
Myasthenia Gravis: A Manual for the Health Care Provider (PDF), page 25.
-Mark-